1
h2. A Guide to Active Record Associations
3
This guide covers the association features of Active Record. By referring to this guide, you will be able to:
5
* Declare associations between Active Record models
6
* Understand the various types of Active Record associations
7
* Use the methods added to your models by creating associations
13
Why do we need associations between models? Because they make common operations simpler and easier in your code. For example, consider a simple Rails application that includes a model for customers and a model for orders. Each customer can have many orders. Without associations, the model declarations would look like this:
16
class Customer < ActiveRecord::Base
19
class Order < ActiveRecord::Base
23
Now, suppose we wanted to add a new order for an existing customer. We'd need to do something like this:
26
@order = Order.create(:order_date => Time.now,
27
:customer_id => @customer.id)
30
Or consider deleting a customer, and ensuring that all of its orders get deleted as well:
33
@orders = Order.find_by_customer_id(@customer.id)
34
@orders.each do |order|
40
With Active Record associations, we can streamline these -- and other -- operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up customers and orders:
43
class Customer < ActiveRecord::Base
44
has_many :orders, :dependent => :destroy
47
class Order < ActiveRecord::Base
52
With this change, creating a new order for a particular customer is easier:
55
@order = @customer.orders.create(:order_date => Time.now)
58
Deleting a customer and all of its orders is _much_ easier:
64
To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails.
66
h3. The Types of Associations
68
In Rails, an _association_ is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model +belongs_to+ another, you instruct Rails to maintain Primary Key–Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model. Rails supports six types of association:
75
* +has_and_belongs_to_many+
77
In the remainder of this guide, you'll learn how to declare and use the various forms of associations. But first, a quick introduction to the situations where each association type is appropriate.
79
h4. The +belongs_to+ Association
81
A +belongs_to+ association sets up a one-to-one connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes customers and orders, and each order can be assigned to exactly one customer, you'd declare the order model this way:
84
class Order < ActiveRecord::Base
89
!images/belongs_to.png(belongs_to Association Diagram)!
91
h4. The +has_one+ Association
93
A +has_one+ association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences). This association indicates that each instance of a model contains or possesses one instance of another model. For example, if each supplier in your application has only one account, you'd declare the supplier model like this:
96
class Supplier < ActiveRecord::Base
101
!images/has_one.png(has_one Association Diagram)!
103
h4. The +has_many+ Association
105
A +has_many+ association indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a +belongs_to+ association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing customers and orders, the customer model could be declared like this:
108
class Customer < ActiveRecord::Base
113
NOTE: The name of the other model is pluralized when declaring a +has_many+ association.
115
!images/has_many.png(has_many Association Diagram)!
117
h4. The +has_many :through+ Association
119
A +has_many :through+ association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding _through_ a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:
122
class Physician < ActiveRecord::Base
123
has_many :appointments
124
has_many :patients, :through => :appointments
127
class Appointment < ActiveRecord::Base
128
belongs_to :physician
132
class Patient < ActiveRecord::Base
133
has_many :appointments
134
has_many :physicians, :through => :appointments
138
!images/has_many_through.png(has_many :through Association Diagram)!
140
The +has_many :through+ association is also useful for setting up "shortcuts" through nested +has_many+ associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:
143
class Document < ActiveRecord::Base
145
has_many :paragraphs, :through => :sections
148
class Section < ActiveRecord::Base
153
class Paragraph < ActiveRecord::Base
158
h4. The +has_one :through+ Association
160
A +has_one :through+ association sets up a one-to-one connection with another model. This association indicates that the declaring model can be matched with one instance of another model by proceeding _through_ a third model. For example, if each supplier has one account, and each account is associated with one account history, then the customer model could look like this:
163
class Supplier < ActiveRecord::Base
165
has_one :account_history, :through => :account
168
class Account < ActiveRecord::Base
170
has_one :account_history
173
class AccountHistory < ActiveRecord::Base
178
!images/has_one_through.png(has_one :through Association Diagram)!
180
h4. The +has_and_belongs_to_many+ Association
182
A +has_and_belongs_to_many+ association creates a direct many-to-many connection with another model, with no intervening model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
185
class Assembly < ActiveRecord::Base
186
has_and_belongs_to_many :parts
189
class Part < ActiveRecord::Base
190
has_and_belongs_to_many :assemblies
194
!images/habtm.png(has_and_belongs_to_many Association Diagram)!
196
h4. Choosing Between +belongs_to+ and +has_one+
198
If you want to set up a 1–1 relationship between two models, you'll need to add +belongs_to+ to one, and +has_one+ to the other. How do you know which is which?
200
The distinction is in where you place the foreign key (it goes on the table for the class declaring the +belongs_to+ association), but you should give some thought to the actual meaning of the data as well. The +has_one+ relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this:
203
class Supplier < ActiveRecord::Base
207
class Account < ActiveRecord::Base
212
The corresponding migration might look like this:
215
class CreateSuppliers < ActiveRecord::Migration
217
create_table :suppliers do |t|
222
create_table :accounts do |t|
223
t.integer :supplier_id
224
t.string :account_number
231
drop_table :suppliers
236
NOTE: Using +t.integer :supplier_id+ makes the foreign key naming obvious and explicit. In current versions of Rails, you can abstract away this implementation detail by using +t.references :supplier+ instead.
238
h4. Choosing Between +has_many :through+ and +has_and_belongs_to_many+
240
Rails offers two different ways to declare a many-to-many relationship between models. The simpler way is to use +has_and_belongs_to_many+, which allows you to make the association directly:
243
class Assembly < ActiveRecord::Base
244
has_and_belongs_to_many :parts
247
class Part < ActiveRecord::Base
248
has_and_belongs_to_many :assemblies
252
The second way to declare a many-to-many relationship is to use +has_many :through+. This makes the association indirectly, through a join model:
255
class Assembly < ActiveRecord::Base
257
has_many :parts, :through => :manifests
260
class Manifest < ActiveRecord::Base
265
class Part < ActiveRecord::Base
267
has_many :assemblies, :through => :manifests
271
The simplest rule of thumb is that you should set up a +has_many :through+ relationship if you need to work with the relationship model as an independent entity. If you don't need to do anything with the relationship model, it may be simpler to set up a +has_and_belongs_to_many+ relationship (though you'll need to remember to create the joining table in the database).
273
You should use +has_many :through+ if you need validations, callbacks, or extra attributes on the join model.
275
h4. Polymorphic Associations
277
A slightly more advanced twist on associations is the _polymorphic association_. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared:
280
class Picture < ActiveRecord::Base
281
belongs_to :imageable, :polymorphic => true
284
class Employee < ActiveRecord::Base
285
has_many :pictures, :as => :imageable
288
class Product < ActiveRecord::Base
289
has_many :pictures, :as => :imageable
293
You can think of a polymorphic +belongs_to+ declaration as setting up an interface that any other model can use. From an instance of the +Employee+ model, you can retrieve a collection of pictures: +@employee.pictures+.
295
Similarly, you can retrieve +@product.pictures+.
297
If you have an instance of the +Picture+ model, you can get to its parent via +@picture.imageable+. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:
300
class CreatePictures < ActiveRecord::Migration
302
create_table :pictures do |t|
304
t.integer :imageable_id
305
t.string :imageable_type
316
This migration can be simplified by using the +t.references+ form:
319
class CreatePictures < ActiveRecord::Migration
321
create_table :pictures do |t|
323
t.references :imageable, :polymorphic => true
334
!images/polymorphic.png(Polymorphic Association Diagram)!
338
In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:
341
class Employee < ActiveRecord::Base
342
has_many :subordinates, :class_name => "Employee",
343
:foreign_key => "manager_id"
344
belongs_to :manager, :class_name => "Employee"
348
With this setup, you can retrieve +@employee.subordinates+ and +@employee.manager+.
350
h3. Tips, Tricks, and Warnings
352
Here are a few things you should know to make efficient use of Active Record associations in your Rails applications:
354
* Controlling caching
355
* Avoiding name collisions
356
* Updating the schema
357
* Controlling association scope
359
h4. Controlling Caching
361
All of the association methods are built around caching, which keeps the result of the most recent query available for further operations. The cache is even shared across methods. For example:
364
customer.orders # retrieves orders from the database
365
customer.orders.size # uses the cached copy of orders
366
customer.orders.empty? # uses the cached copy of orders
369
But what if you want to reload the cache, because data might have been changed by some other part of the application? Just pass +true+ to the association call:
372
customer.orders # retrieves orders from the database
373
customer.orders.size # uses the cached copy of orders
374
customer.orders(true).empty? # discards the cached copy of orders
375
# and goes back to the database
378
h4. Avoiding Name Collisions
380
You are not free to use just any name for your associations. Because creating an association adds a method with that name to the model, it is a bad idea to give an association a name that is already used for an instance method of +ActiveRecord::Base+. The association method would override the base method and break things. For instance, +attributes+ or +connection+ are bad names for associations.
382
h4. Updating the Schema
384
Associations are extremely useful, but they are not magic. You are responsible for maintaining your database schema to match your associations. In practice, this means two things, depending on what sort of associations you are creating. For +belongs_to+ associations you need to create foreign keys, and for +has_and_belongs_to_many+ associations you need to create the appropriate join table.
386
h5. Creating Foreign Keys for +belongs_to+ Associations
388
When you declare a +belongs_to+ association, you need to create foreign keys as appropriate. For example, consider this model:
391
class Order < ActiveRecord::Base
396
This declaration needs to be backed up by the proper foreign key declaration on the orders table:
399
class CreateOrders < ActiveRecord::Migration
401
create_table :orders do |t|
402
t.datetime :order_date
403
t.string :order_number
404
t.integer :customer_id
414
If you create an association some time after you build the underlying model, you need to remember to create an +add_column+ migration to provide the necessary foreign key.
416
h5. Creating Join Tables for +has_and_belongs_to_many+ Associations
418
If you create a +has_and_belongs_to_many+ association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the +:join_table+ option, Active Record creates the name by using the lexical order of the class names. So a join between customer and order models will give the default join table name of "customers_orders" because "c" outranks "o" in lexical ordering.
420
WARNING: The precedence between model names is calculated using the +<+ operator for +String+. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically _less_ than 's' in common encodings).
422
Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations:
425
class Assembly < ActiveRecord::Base
426
has_and_belongs_to_many :parts
429
class Part < ActiveRecord::Base
430
has_and_belongs_to_many :assemblies
434
These need to be backed up by a migration to create the +assemblies_parts+ table. This table should be created without a primary key:
437
class CreateAssemblyPartJoinTable < ActiveRecord::Migration
439
create_table :assemblies_parts, :id => false do |t|
440
t.integer :assembly_id
446
drop_table :assemblies_parts
451
We pass +:id => false+ to +create_table+ because that table does not represent a model. That's required for the association to work properly. If you observe any strange behaviour in a +has_and_belongs_to_many+ association like mangled models IDs, or exceptions about conflicting IDs chances are you forgot that bit.
453
h4. Controlling Association Scope
455
By default, associations look for objects only within the current module's scope. This can be important when you declare Active Record models within a module. For example:
460
class Supplier < ActiveRecord::Base
464
class Account < ActiveRecord::Base
471
This will work fine, because both the +Supplier+ and the +Account+ class are defined within the same scope. But the following will _not_ work, because +Supplier+ and +Account+ are defined in different scopes:
476
class Supplier < ActiveRecord::Base
482
class Account < ActiveRecord::Base
489
To associate a model with a model in a different namespace, you must specify the complete class name in your association declaration:
494
class Supplier < ActiveRecord::Base
496
:class_name => "MyApplication::Billing::Account"
501
class Account < ActiveRecord::Base
502
belongs_to :supplier,
503
:class_name => "MyApplication::Business::Supplier"
509
h3. Detailed Association Reference
511
The following sections give the details of each type of association, including the methods that they add and the options that you can use when declaring an association.
513
h4. +belongs_to+ Association Reference
515
The +belongs_to+ association creates a one-to-one match with another model. In database terms, this association says that this class contains the foreign key. If the other class contains the foreign key, then you should use +has_one+ instead.
517
h5. Methods Added by +belongs_to+
519
When you declare a +belongs_to+ association, the declaring class automatically gains four methods related to the association:
521
* <tt><em>association</em>(force_reload = false)</tt>
522
* <tt><em>association</em>=(associate)</tt>
523
* <tt>build_<em>association</em>(attributes = {})</tt>
524
* <tt>create_<em>association</em>(attributes = {})</tt>
526
In all of these methods, <tt><em>association</em></tt> is replaced with the symbol passed as the first argument to +belongs_to+. For example, given the declaration:
529
class Order < ActiveRecord::Base
534
Each instance of the order model will have these methods:
543
h6. _association_(force_reload = false)
545
The <tt><em>association</em></tt> method returns the associated object, if any. If no associated object is found, it returns +nil+.
548
@customer = @order.customer
551
If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), pass +true+ as the +force_reload+ argument.
553
h6. _association_=(associate)
555
The <tt><em>association</em>=</tt> method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associate object and setting this object's foreign key to the same value.
558
@order.customer = @customer
561
h6. build_<em>association</em>(attributes = {})
563
The <tt>build_<em>association</em></tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set, but the associated object will _not_ yet be saved.
566
@customer = @order.build_customer(:customer_number => 123,
567
:customer_name => "John Doe")
570
h6. create_<em>association</em>(attributes = {})
572
The <tt>create_<em>association</em></tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set. In addition, the associated object _will_ be saved (assuming that it passes any validations).
575
@customer = @order.create_customer(:customer_number => 123,
576
:customer_name => "John Doe")
580
h5. Options for +belongs_to+
582
In many situations, you can use the default behavior of +belongs_to+ without any customization. But despite Rails' emphasis of convention over customization, you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a +belongs_to+ association. For example, an association with several options might look like this:
585
class Order < ActiveRecord::Base
586
belongs_to :customer, :counter_cache => true,
587
:conditions => "active = 1"
591
The +belongs_to+ association supports these options:
607
If you set the +:autosave+ option to +true+, Rails will save any loaded members and destroy members that are marked for destruction whenever you save the parent object.
611
If the name of the other model cannot be derived from the association name, you can use the +:class_name+ option to supply the model name. For example, if an order belongs to a customer, but the actual name of the model containing customers is +Patron+, you'd set things up this way:
614
class Order < ActiveRecord::Base
615
belongs_to :customer, :class_name => "Patron"
621
The +:conditions+ option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL +WHERE+ clause).
624
class Order < ActiveRecord::Base
625
belongs_to :customer, :conditions => "active = 1"
631
The +:counter_cache+ option can be used to make finding the number of belonging objects more efficient. Consider these models:
634
class Order < ActiveRecord::Base
637
class Customer < ActiveRecord::Base
642
With these declarations, asking for the value of +@customer.orders.size+ requires making a call to the database to perform a +COUNT(*)+ query. To avoid this call, you can add a counter cache to the _belonging_ model:
645
class Order < ActiveRecord::Base
646
belongs_to :customer, :counter_cache => true
648
class Customer < ActiveRecord::Base
653
With this declaration, Rails will keep the cache value up to date, and then return that value in response to the +size+ method.
655
Although the +:counter_cache+ option is specified on the model that includes the +belongs_to+ declaration, the actual column must be added to the _associated_ model. In the case above, you would need to add a column named +orders_count+ to the +Customer+ model. You can override the default column name if you need to:
658
class Order < ActiveRecord::Base
659
belongs_to :customer, :counter_cache => :count_of_orders
661
class Customer < ActiveRecord::Base
666
Counter cache columns are added to the containing model's list of read-only attributes through +attr_readonly+.
670
If you set the +:dependent+ option to +:destroy+, then deleting this object will call the +destroy+ method on the associated object to delete that object. If you set the +:dependent+ option to +:delete+, then deleting this object will delete the associated object _without_ calling its +destroy+ method.
672
WARNING: You should not specify this option on a +belongs_to+ association that is connected with a +has_many+ association on the other class. Doing so can lead to orphaned records in your database.
676
By convention, Rails guesses that the column used to hold the foreign key on this model is the name of the association with the suffix +_id+ added. The +:foreign_key+ option lets you set the name of the foreign key directly:
679
class Order < ActiveRecord::Base
680
belongs_to :customer, :class_name => "Patron",
681
:foreign_key => "patron_id"
685
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
689
You can use the +:include+ option to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
692
class LineItem < ActiveRecord::Base
696
class Order < ActiveRecord::Base
701
class Customer < ActiveRecord::Base
706
If you frequently retrieve customers directly from line items (+@line_item.order.customer+), then you can make your code somewhat more efficient by including customers in the association from line items to orders:
709
class LineItem < ActiveRecord::Base
710
belongs_to :order, :include => :customer
713
class Order < ActiveRecord::Base
718
class Customer < ActiveRecord::Base
723
NOTE: There's no need to use +:include+ for immediate associations - that is, if you have +Order belongs_to :customer+, then the customer is eager-loaded automatically when it's needed.
727
Passing +true+ to the +:polymorphic+ option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail <a href="#polymorphic-associations">earlier in this guide</a>.
731
If you set the +:readonly+ option to +true+, then the associated object will be read-only when retrieved via the association.
735
The +:select+ option lets you override the SQL +SELECT+ clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
737
TIP: If you set the +:select+ option on a +belongs_to+ association, you should also set the +foreign_key+ option to guarantee the correct results.
741
If you set the +:validate+ option to +true+, then associated objects will be validated whenever you save this object. By default, this is +false+: associated objects will not be validated when this object is saved.
743
h5. How To Know Whether There's an Associated Object?
745
To know whether there's and associated object just check <tt><em>association</em>.nil?</tt>:
748
if @order.customer.nil?
749
@msg = "No customer found for this order"
753
h5. When are Objects Saved?
755
Assigning an object to a +belongs_to+ association does _not_ automatically save the object. It does not save the associated object either.
757
h4. +has_one+ Association Reference
759
The +has_one+ association creates a one-to-one match with another model. In database terms, this association says that the other class contains the foreign key. If this class contains the foreign key, then you should use +belongs_to+ instead.
761
h5. Methods Added by +has_one+
763
When you declare a +has_one+ association, the declaring class automatically gains four methods related to the association:
765
* <tt><em>association</em>(force_reload = false)</tt>
766
* <tt><em>association</em>=(associate)</tt>
767
* <tt>build_<em>association</em>(attributes = {})</tt>
768
* <tt>create_<em>association</em>(attributes = {})</tt>
770
In all of these methods, <tt><em>association</em></tt> is replaced with the symbol passed as the first argument to +has_one+. For example, given the declaration:
773
class Supplier < ActiveRecord::Base
778
Each instance of the +Supplier+ model will have these methods:
787
h6. <tt><em>association</em>(force_reload = false)</tt>
789
The <tt><em>association</em></tt> method returns the associated object, if any. If no associated object is found, it returns +nil+.
792
@account = @supplier.account
795
If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), pass +true+ as the +force_reload+ argument.
797
h6. <tt><em>association</em>=(associate)</tt>
799
The <tt><em>association</em>=</tt> method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from this object and setting the associate object's foreign key to the same value.
802
@supplier.account = @account
805
h6. <tt>build_<em>association</em>(attributes = {})</tt>
807
The <tt>build_<em>association</em></tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through its foreign key will be set, but the associated object will _not_ yet be saved.
810
@account = @supplier.build_account(:terms => "Net 30")
813
h6. <tt>create_<em>association</em>(attributes = {})</tt>
815
The <tt>create_<em>association</em></tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through its foreign key will be set. In addition, the associated object _will_ be saved (assuming that it passes any validations).
818
@account = @supplier.create_account(:terms => "Net 30")
821
h5. Options for +has_one+
823
In many situations, you can use the default behavior of +has_one+ without any customization. But despite Rails' emphasis of convention over customization, you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a +has_one+ association. For example, an association with several options might look like this:
826
class Supplier < ActiveRecord::Base
827
has_one :account, :class_name => "Billing", :dependent => :nullify
831
The +has_one+ association supports these options:
851
Setting the +:as+ option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail <a href="#polymorphic-associations">earlier in this guide</a>.
855
If you set the +:autosave+ option to +true+, Rails will save any loaded members and destroy members that are marked for destruction whenever you save the parent object.
859
If the name of the other model cannot be derived from the association name, you can use the +:class_name+ option to supply the model name. For example, if a supplier has an account, but the actual name of the model containing accounts is +Billing+, you'd set things up this way:
862
class Supplier < ActiveRecord::Base
863
has_one :account, :class_name => "Billing"
869
The +:conditions+ option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL +WHERE+ clause).
872
class Supplier < ActiveRecord::Base
873
has_one :account, :conditions => "confirmed = 1"
879
If you set the +:dependent+ option to +:destroy+, then deleting this object will call the +destroy+ method on the associated object to delete that object. If you set the +:dependent+ option to +:delete+, then deleting this object will delete the associated object _without_ calling its +destroy+ method. If you set the +:dependent+ option to +:nullify+, then deleting this object will set the foreign key in the association object to +NULL+.
883
By convention, Rails guesses that the column used to hold the foreign key on the other model is the name of this model with the suffix +_id+ added. The +:foreign_key+ option lets you set the name of the foreign key directly:
886
class Supplier < ActiveRecord::Base
887
has_one :account, :foreign_key => "supp_id"
891
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
895
You can use the +:include+ option to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
898
class Supplier < ActiveRecord::Base
902
class Account < ActiveRecord::Base
904
belongs_to :representative
907
class Representative < ActiveRecord::Base
912
If you frequently retrieve representatives directly from suppliers (+@supplier.account.representative+), then you can make your code somewhat more efficient by including representatives in the association from suppliers to accounts:
915
class Supplier < ActiveRecord::Base
916
has_one :account, :include => :representative
919
class Account < ActiveRecord::Base
921
belongs_to :representative
924
class Representative < ActiveRecord::Base
931
The +:order+ option dictates the order in which associated objects will be received (in the syntax used by a SQL +ORDER BY+ clause). Because a +has_one+ association will only retrieve a single associated object, this option should not be needed.
935
By convention, Rails guesses that the column used to hold the primary key of this model is +id+. You can override this and explicitly specify the primary key with the +:primary_key+ option.
939
If you set the +:readonly+ option to +true+, then the associated object will be read-only when retrieved via the association.
943
The +:select+ option lets you override the SQL +SELECT+ clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
947
The +:source+ option specifies the source association name for a +has_one :through+ association.
951
The +:source_type+ option specifies the source association type for a +has_one :through+ association that proceeds through a polymorphic association.
955
The +:through+ option specifies a join model through which to perform the query. +has_one :through+ associations were discussed in detail <a href="#the-has-one-through-association">earlier in this guide</a>.
959
If you set the +:validate+ option to +true+, then associated objects will be validated whenever you save this object. By default, this is +false+: associated objects will not be validated when this object is saved.
961
h5. How To Know Whether There's an Associated Object?
963
To know whether there's and associated object just check <tt><em>association</em>.nil?</tt>:
966
if @supplier.account.nil?
967
@msg = "No account found for this supplier"
971
h5. When are Objects Saved?
973
When you assign an object to a +has_one+ association, that object is automatically saved (in order to update its foreign key). In addition, any object being replaced is also automatically saved, because its foreign key will change too.
975
If either of these saves fails due to validation errors, then the assignment statement returns +false+ and the assignment itself is cancelled.
977
If the parent object (the one declaring the +has_one+ association) is unsaved (that is, +new_record?+ returns +true+) then the child objects are not saved. They will automatically when the parent object is saved.
979
If you want to assign an object to a +has_one+ association without saving the object, use the <tt><em>association</em>.build</tt> method.
981
h4. +has_many+ Association Reference
983
The +has_many+ association creates a one-to-many relationship with another model. In database terms, this association says that the other class will have a foreign key that refers to instances of this class.
987
When you declare a +has_many+ association, the declaring class automatically gains 13 methods related to the association:
989
* <tt><em>collection</em>(force_reload = false)</tt>
990
* <tt><em>collection</em><<(object, ...)</tt>
991
* <tt><em>collection</em>.delete(object, ...)</tt>
992
* <tt><em>collection</em>=objects</tt>
993
* <tt><em>collection_singular</em>_ids</tt>
994
* <tt><em>collection_singular</em>_ids=ids</tt>
995
* <tt><em>collection</em>.clear</tt>
996
* <tt><em>collection</em>.empty?</tt>
997
* <tt><em>collection</em>.size</tt>
998
* <tt><em>collection</em>.find(...)</tt>
999
* <tt><em>collection</em>.exist?(...)</tt>
1000
* <tt><em>collection</em>.build(attributes = {}, ...)</tt>
1001
* <tt><em>collection</em>.create(attributes = {})</tt>
1003
In all of these methods, <tt><em>collection</em></tt> is replaced with the symbol passed as the first argument to +has_many+, and <tt><em>collection_singular</em></tt> is replaced with the singularized version of that symbol.. For example, given the declaration:
1006
class Customer < ActiveRecord::Base
1011
Each instance of the customer model will have these methods:
1014
orders(force_reload = false)
1015
orders<<(object, ...)
1016
orders.delete(object, ...)
1025
orders.build(attributes = {}, ...)
1026
orders.create(attributes = {})
1029
h6. <tt><em>collection</em>(force_reload = false)</tt>
1031
The <tt><em>collection</em></tt> method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array.
1034
@orders = @customer.orders
1037
h6. <tt><em>collection</em><<(object, ...)</tt>
1039
The <tt><em>collection</em><<</tt> method adds one or more objects to the collection by setting their foreign keys to the primary key of the calling model.
1042
@customer.orders << @order1
1045
h6. <tt><em>collection</em>.delete(object, ...)</tt>
1047
The <tt><em>collection</em>.delete</tt> method removes one or more objects from the collection by setting their foreign keys to +NULL+.
1050
@customer.orders.delete(@order1)
1053
WARNING: Objects will be in addition destroyed if they're associated with +:dependent => :destroy+, and deleted if they're associated with +:dependent => :delete_all+.
1056
h6. <tt><em>collection</em>=objects</tt>
1058
The <tt><em>collection</em>=</tt> method makes the collection contain only the supplied objects, by adding and deleting as appropriate.
1060
h6. <tt><em>collection_singular</em>_ids</tt>
1062
The <tt><em>collection_singular</em>_ids</tt> method returns an array of the ids of the objects in the collection.
1065
@order_ids = @customer.order_ids
1068
h6. <tt><em>collection_singular</em>_ids=ids</tt>
1070
The <tt><em>collection_singular</em>_ids=</tt> method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate.
1072
h6. <tt><em>collection</em>.clear</tt>
1074
The <tt><em>collection</em>.clear</tt> method removes every object from the collection. This destroys the associated objects if they are associated with +:dependent => :destroy+, deletes them directly from the database if +:dependent => :delete_all+, and otherwise sets their foreign keys to +NULL+.
1076
h6. <tt><em>collection</em>.empty?</tt>
1078
The <tt><em>collection</em>.empty?</tt> method returns +true+ if the collection does not contain any associated objects.
1081
<% if @customer.orders.empty? %>
1086
h6. <tt><em>collection</em>.size</tt>
1088
The <tt><em>collection</em>.size</tt> method returns the number of objects in the collection.
1091
@order_count = @customer.orders.size
1094
h6. <tt><em>collection</em>.find(...)</tt>
1096
The <tt><em>collection</em>.find</tt> method finds objects within the collection. It uses the same syntax and options as +ActiveRecord::Base.find+.
1099
@open_orders = @customer.orders.find(:all, :conditions => "open = 1")
1102
h6. <tt><em>collection</em>.exist?(...)</tt>
1104
The <tt><em>collection</em>.exist?</tt> method checks whether an object meeting the supplied conditions exists in the collection. It uses the same syntax and options as +ActiveRecord::Base.exists?+.
1106
h6. <tt><em>collection</em>.build(attributes = {}, ...)</tt>
1108
The <tt><em>collection</em>.build</tt> method returns one or more new objects of the associated type. These objects will be instantiated from the passed attributes, and the link through their foreign key will be created, but the associated objects will _not_ yet be saved.
1111
@order = @customer.orders.build(:order_date => Time.now,
1112
:order_number => "A12345")
1115
h6. <tt><em>collection</em>.create(attributes = {})</tt>
1117
The <tt><em>collection</em>.create</tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through its foreign key will be created, and the associated object _will_ be saved (assuming that it passes any validations).
1120
@order = @customer.orders.create(:order_date => Time.now,
1121
:order_number => "A12345")
1124
h5. Options for +has_many+
1126
In many situations, you can use the default behavior for +has_many+ without any customization. But you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a +has_many+ association. For example, an association with several options might look like this:
1129
class Customer < ActiveRecord::Base
1130
has_many :orders, :dependent => :delete_all, :validate => :false
1134
The +has_many+ association supports these options:
1161
Setting the +:as+ option indicates that this is a polymorphic association, as discussed <a href="#polymorphic-associations">earlier in this guide</a>.
1165
If you set the +:autosave+ option to +true+, Rails will save any loaded members and destroy members that are marked for destruction whenever you save the parent object.
1169
If the name of the other model cannot be derived from the association name, you can use the +:class_name+ option to supply the model name. For example, if a customer has many orders, but the actual name of the model containing orders is +Transaction+, you'd set things up this way:
1172
class Customer < ActiveRecord::Base
1173
has_many :orders, :class_name => "Transaction"
1179
The +:conditions+ option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL +WHERE+ clause).
1182
class Customer < ActiveRecord::Base
1183
has_many :confirmed_orders, :class_name => "Order",
1184
:conditions => "confirmed = 1"
1188
You can also set conditions via a hash:
1191
class Customer < ActiveRecord::Base
1192
has_many :confirmed_orders, :class_name => "Order",
1193
:conditions => { :confirmed => true }
1197
If you use a hash-style +:conditions+ option, then record creation via this association will be automatically scoped using the hash. In this case, using +@customer.confirmed_orders.create+ or +@customer.confirmed_orders.build+ will create orders where the confirmed column has the value +true+.
1201
Normally Rails automatically generates the proper SQL to count the association members. With the +:counter_sql+ option, you can specify a complete SQL statement to count them yourself.
1203
NOTE: If you specify +:finder_sql+ but not +:counter_sql+, then the counter SQL will be generated by substituting +SELECT COUNT(*) FROM+ for the +SELECT ... FROM+ clause of your +:finder_sql+ statement.
1207
If you set the +:dependent+ option to +:destroy+, then deleting this object will call the +destroy+ method on the associated objects to delete those objects. If you set the +:dependent+ option to +:delete_all+, then deleting this object will delete the associated objects _without_ calling their +destroy+ method. If you set the +:dependent+ option to +:nullify+, then deleting this object will set the foreign key in the associated objects to +NULL+.
1209
NOTE: This option is ignored when you use the +:through+ option on the association.
1213
The +:extend+ option specifies a named module to extend the association proxy. Association extensions are discussed in detail <a href="#association-extensions">later in this guide</a>.
1217
Normally Rails automatically generates the proper SQL to fetch the association members. With the +:finder_sql+ option, you can specify a complete SQL statement to fetch them yourself. If fetching objects requires complex multi-table SQL, this may be necessary.
1221
By convention, Rails guesses that the column used to hold the foreign key on the other model is the name of this model with the suffix +_id+ added. The +:foreign_key+ option lets you set the name of the foreign key directly:
1224
class Customer < ActiveRecord::Base
1225
has_many :orders, :foreign_key => "cust_id"
1229
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
1233
The +:group+ option supplies an attribute name to group the result set by, using a +GROUP BY+ clause in the finder SQL.
1236
class Customer < ActiveRecord::Base
1237
has_many :line_items, :through => :orders, :group => "orders.id"
1243
You can use the +:include+ option to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
1246
class Customer < ActiveRecord::Base
1250
class Order < ActiveRecord::Base
1251
belongs_to :customer
1252
has_many :line_items
1255
class LineItem < ActiveRecord::Base
1260
If you frequently retrieve line items directly from customers (+@customer.orders.line_items+), then you can make your code somewhat more efficient by including line items in the association from customers to orders:
1263
class Customer < ActiveRecord::Base
1264
has_many :orders, :include => :line_items
1267
class Order < ActiveRecord::Base
1268
belongs_to :customer
1269
has_many :line_items
1272
class LineItem < ActiveRecord::Base
1279
The +:limit+ option lets you restrict the total number of objects that will be fetched through an association.
1282
class Customer < ActiveRecord::Base
1283
has_many :recent_orders, :class_name => "Order",
1284
:order => "order_date DESC", :limit => 100
1290
The +:offset+ option lets you specify the starting offset for fetching objects via an association. For example, if you set +:offset => 11+, it will skip the first 11 records.
1294
The +:order+ option dictates the order in which associated objects will be received (in the syntax used by a SQL +ORDER BY+ clause).
1297
class Customer < ActiveRecord::Base
1298
has_many :orders, :order => "date_confirmed DESC"
1304
By convention, Rails guesses that the column used to hold the primary key of the association is +id+. You can override this and explicitly specify the primary key with the +:primary_key+ option.
1308
If you set the +:readonly+ option to +true+, then the associated objects will be read-only when retrieved via the association.
1312
The +:select+ option lets you override the SQL +SELECT+ clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
1314
WARNING: If you specify your own +:select+, be sure to include the primary key and foreign key columns of the associated model. If you do not, Rails will throw an error.
1318
The +:source+ option specifies the source association name for a +has_many :through+ association. You only need to use this option if the name of the source association cannot be automatically inferred from the association name.
1322
The +:source_type+ option specifies the source association type for a +has_many :through+ association that proceeds through a polymorphic association.
1326
The +:through+ option specifies a join model through which to perform the query. +has_many :through+ associations provide a way to implement many-to-many relationships, as discussed <a href="#the-has-many-through-association">earlier in this guide</a>.
1330
Specify the +:uniq => true+ option to remove duplicates from the collection. This is most useful in conjunction with the +:through+ option.
1334
If you set the +:validate+ option to +false+, then associated objects will not be validated whenever you save this object. By default, this is +true+: associated objects will be validated when this object is saved.
1336
h5. When are Objects Saved?
1338
When you assign an object to a +has_many+ association, that object is automatically saved (in order to update its foreign key). If you assign multiple objects in one statement, then they are all saved.
1340
If any of these saves fails due to validation errors, then the assignment statement returns +false+ and the assignment itself is cancelled.
1342
If the parent object (the one declaring the +has_many+ association) is unsaved (that is, +new_record?+ returns +true+) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
1344
If you want to assign an object to a +has_many+ association without saving the object, use the <tt><em>collection</em>.build</tt> method.
1346
h4. +has_and_belongs_to_many+ Association Reference
1348
The +has_and_belongs_to_many+ association creates a many-to-many relationship with another model. In database terms, this associates two classes via an intermediate join table that includes foreign keys referring to each of the classes.
1352
When you declare a +has_and_belongs_to_many+ association, the declaring class automatically gains 13 methods related to the association:
1354
* <tt><em>collection</em>(force_reload = false)</tt>
1355
* <tt><em>collection</em><<(object, ...)</tt>
1356
* <tt><em>collection</em>.delete(object, ...)</tt>
1357
* <tt><em>collection</em>=objects</tt>
1358
* <tt><em>collection_singular</em>_ids</tt>
1359
* <tt><em>collection_singular</em>_ids=ids</tt>
1360
* <tt><em>collection</em>.clear</tt>
1361
* <tt><em>collection</em>.empty?</tt>
1362
* <tt><em>collection</em>.size</tt>
1363
* <tt><em>collection</em>.find(...)</tt>
1364
* <tt><em>collection</em>.exist?(...)</tt>
1365
* <tt><em>collection</em>.build(attributes = {})</tt>
1366
* <tt><em>collection</em>.create(attributes = {})</tt>
1368
In all of these methods, <tt><em>collection</em></tt> is replaced with the symbol passed as the first argument to +has_and_belongs_to_many+, and <tt><em>collection_singular</em></tt> is replaced with the singularized version of that symbol.. For example, given the declaration:
1371
class Part < ActiveRecord::Base
1372
has_and_belongs_to_many :assemblies
1376
Each instance of the part model will have these methods:
1379
assemblies(force_reload = false)
1380
assemblies<<(object, ...)
1381
assemblies.delete(object, ...)
1388
assemblies.find(...)
1389
assemblies.exist?(...)
1390
assemblies.build(attributes = {}, ...)
1391
assemblies.create(attributes = {})
1394
h6. Additional Column Methods
1396
If the join table for a +has_and_belongs_to_many+ association has additional columns beyond the two foreign keys, these columns will be added as attributes to records retrieved via that association. Records returned with additional attributes will always be read-only, because Rails cannot save changes to those attributes.
1398
WARNING: The use of extra attributes on the join table in a +has_and_belongs_to_many+ association is deprecated. If you require this sort of complex behavior on the table that joins two models in a many-to-many relationship, you should use a +has_many :through+ association instead of +has_and_belongs_to_many+.
1401
h6. <tt><em>collection</em>(force_reload = false)</tt>
1403
The <tt><em>collection</em></tt> method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array.
1406
@assemblies = @part.assemblies
1409
h6. <tt><em>collection</em><<(object, ...)</tt>
1411
The <tt><em>collection</em><<</tt> method adds one or more objects to the collection by creating records in the join table.
1414
@part.assemblies << @assembly1
1417
NOTE: This method is aliased as <tt><em>collection</em>.concat</tt> and <tt><em>collection</em>.push</tt>.
1419
h6. <tt><em>collection</em>.delete(object, ...)</tt>
1421
The <tt><em>collection</em>.delete</tt> method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.
1424
@part.assemblies.delete(@assembly1)
1427
h6. <tt><em>collection</em>=objects</tt>
1429
The <tt><em>collection</em>=</tt> method makes the collection contain only the supplied objects, by adding and deleting as appropriate.
1431
h6. <tt><em>collection_singular</em>_ids</tt>
1433
The <tt><em>collection_singular</em>_ids</tt> method returns an array of the ids of the objects in the collection.
1436
@assembly_ids = @part.assembly_ids
1439
h6. <tt><em>collection_singular</em>_ids=ids</tt>
1441
The <tt><em>collection_singular</em>_ids=</tt> method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate.
1443
h6. <tt><em>collection</em>.clear</tt>
1445
The <tt><em>collection</em>.clear</tt> method removes every object from the collection by deleting the rows from the joining table. This does not destroy the associated objects.
1447
h6. <tt><em>collection</em>.empty?</tt>
1449
The <tt><em>collection</em>.empty?</tt> method returns +true+ if the collection does not contain any associated objects.
1452
<% if @part.assemblies.empty? %>
1453
This part is not used in any assemblies
1457
h6. <tt><em>collection</em>.size</tt>
1459
The <tt><em>collection</em>.size</tt> method returns the number of objects in the collection.
1462
@assembly_count = @part.assemblies.size
1465
h6. <tt><em>collection</em>.find(...)</tt>
1467
The <tt><em>collection</em>.find</tt> method finds objects within the collection. It uses the same syntax and options as +ActiveRecord::Base.find+. It also adds the additional condition that the object must be in the collection.
1470
@new_assemblies = @part.assemblies.find(:all,
1471
:conditions => ["created_at > ?", 2.days.ago])
1474
h6. <tt><em>collection</em>.exist?(...)</tt>
1476
The <tt><em>collection</em>.exist?</tt> method checks whether an object meeting the supplied conditions exists in the collection. It uses the same syntax and options as +ActiveRecord::Base.exists?+.
1478
h6. <tt><em>collection</em>.build(attributes = {})</tt>
1480
The <tt><em>collection</em>.build</tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through the join table will be created, but the associated object will _not_ yet be saved.
1483
@assembly = @part.assemblies.build(
1484
{:assembly_name => "Transmission housing"})
1487
h6. <tt><em>collection</em>.create(attributes = {})</tt>
1489
The <tt><em>collection</em>.create</tt> method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through the join table will be created, and the associated object _will_ be saved (assuming that it passes any validations).
1492
@assembly = @part.assemblies.create(
1493
{:assembly_name => "Transmission housing"})
1496
h5. Options for +has_and_belongs_to_many+
1498
In many situations, you can use the default behavior for +has_and_belongs_to_many+ without any customization. But you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a +has_and_belongs_to_many+ association. For example, an association with several options might look like this:
1501
class Parts < ActiveRecord::Base
1502
has_and_belongs_to_many :assemblies, :uniq => true,
1507
The +has_and_belongs_to_many+ association supports these options:
1509
* +:association_foreign_key+
1530
h6. +:association_foreign_key+
1532
By convention, Rails guesses that the column in the join table used to hold the foreign key pointing to the other model is the name of that model with the suffix +_id+ added. The +:association_foreign_key+ option lets you set the name of the foreign key directly:
1534
TIP: The +:foreign_key+ and +:association_foreign_key+ options are useful when setting up a many-to-many self-join. For example:
1537
class User < ActiveRecord::Base
1538
has_and_belongs_to_many :friends, :class_name => "User",
1539
:foreign_key => "this_user_id",
1540
:association_foreign_key => "other_user_id"
1546
If you set the +:autosave+ option to +true+, Rails will save any loaded members and destroy members that are marked for destruction whenever you save the parent object.
1550
If the name of the other model cannot be derived from the association name, you can use the +:class_name+ option to supply the model name. For example, if a part has many assemblies, but the actual name of the model containing assemblies is +Gadget+, you'd set things up this way:
1553
class Parts < ActiveRecord::Base
1554
has_and_belongs_to_many :assemblies, :class_name => "Gadget"
1560
The +:conditions+ option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL +WHERE+ clause).
1563
class Parts < ActiveRecord::Base
1564
has_and_belongs_to_many :assemblies,
1565
:conditions => "factory = 'Seattle'"
1569
You can also set conditions via a hash:
1572
class Parts < ActiveRecord::Base
1573
has_and_belongs_to_many :assemblies,
1574
:conditions => { :factory => 'Seattle' }
1578
If you use a hash-style +:conditions+ option, then record creation via this association will be automatically scoped using the hash. In this case, using +@parts.assemblies.create+ or +@parts.assemblies.build+ will create orders where the +factory+ column has the value "Seattle".
1582
Normally Rails automatically generates the proper SQL to count the association members. With the +:counter_sql+ option, you can specify a complete SQL statement to count them yourself.
1584
NOTE: If you specify +:finder_sql+ but not +:counter_sql+, then the counter SQL will be generated by substituting +SELECT COUNT(*) FROM+ for the +SELECT ... FROM+ clause of your +:finder_sql+ statement.
1588
Normally Rails automatically generates the proper SQL to remove links between the associated classes. With the +:delete_sql+ option, you can specify a complete SQL statement to delete them yourself.
1592
The +:extend+ option specifies a named module to extend the association proxy. Association extensions are discussed in detail <a href="#association-extensions">later in this guide</a>.
1596
Normally Rails automatically generates the proper SQL to fetch the association members. With the +:finder_sql+ option, you can specify a complete SQL statement to fetch them yourself. If fetching objects requires complex multi-table SQL, this may be necessary.
1600
By convention, Rails guesses that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix +_id+ added. The +:foreign_key+ option lets you set the name of the foreign key directly:
1603
class User < ActiveRecord::Base
1604
has_and_belongs_to_many :friends, :class_name => "User",
1605
:foreign_key => "this_user_id",
1606
:association_foreign_key => "other_user_id"
1612
The +:group+ option supplies an attribute name to group the result set by, using a +GROUP BY+ clause in the finder SQL.
1615
class Parts < ActiveRecord::Base
1616
has_and_belongs_to_many :assemblies, :group => "factory"
1622
You can use the +:include+ option to specify second-order associations that should be eager-loaded when this association is used.
1626
Normally Rails automatically generates the proper SQL to create links between the associated classes. With the +:insert_sql+ option, you can specify a complete SQL statement to insert them yourself.
1630
If the default name of the join table, based on lexical ordering, is not what you want, you can use the +:join_table+ option to override the default.
1634
The +:limit+ option lets you restrict the total number of objects that will be fetched through an association.
1637
class Parts < ActiveRecord::Base
1638
has_and_belongs_to_many :assemblies, :order => "created_at DESC",
1645
The +:offset+ option lets you specify the starting offset for fetching objects via an association. For example, if you set +:offset => 11+, it will skip the first 11 records.
1649
The +:order+ option dictates the order in which associated objects will be received (in the syntax used by a SQL +ORDER BY+ clause).
1652
class Parts < ActiveRecord::Base
1653
has_and_belongs_to_many :assemblies, :order => "assembly_name ASC"
1659
If you set the +:readonly+ option to +true+, then the associated objects will be read-only when retrieved via the association.
1663
The +:select+ option lets you override the SQL +SELECT+ clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
1667
Specify the +:uniq => true+ option to remove duplicates from the collection.
1671
If you set the +:validate+ option to +false+, then associated objects will not be validated whenever you save this object. By default, this is +true+: associated objects will be validated when this object is saved.
1673
h5. When are Objects Saved?
1675
When you assign an object to a +has_and_belongs_to_many+ association, that object is automatically saved (in order to update the join table). If you assign multiple objects in one statement, then they are all saved.
1677
If any of these saves fails due to validation errors, then the assignment statement returns +false+ and the assignment itself is cancelled.
1679
If the parent object (the one declaring the +has_and_belongs_to_many+ association) is unsaved (that is, +new_record?+ returns +true+) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
1681
If you want to assign an object to a +has_and_belongs_to_many+ association without saving the object, use the <tt><em>collection</em>.build</tt> method.
1683
h4. Association Callbacks
1685
Normal callbacks hook into the lifecycle of Active Record objects, allowing you to work with those objects at various points. For example, you can use a +:before_save+ callback to cause something to happen just before an object is saved.
1687
Association callbacks are similar to normal callbacks, but they are triggered by events in the lifecycle of a collection. There are four available association callbacks:
1694
You define association callbacks by adding options to the association declaration. For example:
1697
class Customer < ActiveRecord::Base
1698
has_many :orders, :before_add => :check_credit_limit
1700
def check_credit_limit(order)
1706
Rails passes the object being added or removed to the callback.
1708
You can stack callbacks on a single event by passing them as an array:
1711
class Customer < ActiveRecord::Base
1713
:before_add => [:check_credit_limit, :calculate_shipping_charges]
1715
def check_credit_limit(order)
1719
def calculate_shipping_charges(order)
1725
If a +before_add+ callback throws an exception, the object does not get added to the collection. Similarly, if a +before_remove+ callback throws an exception, the object does not get removed from the collection.
1727
h4. Association Extensions
1729
You're not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example:
1732
class Customer < ActiveRecord::Base
1734
def find_by_order_prefix(order_number)
1735
find_by_region_id(order_number[0..2])
1741
If you have an extension that should be shared by many associations, you can use a named extension module. For example:
1744
module FindRecentExtension
1746
find(:all, :conditions => ["created_at > ?", 5.days.ago])
1750
class Customer < ActiveRecord::Base
1751
has_many :orders, :extend => FindRecentExtension
1754
class Supplier < ActiveRecord::Base
1755
has_many :deliveries, :extend => FindRecentExtension
1759
To include more than one extension module in a single association, specify an array of modules:
1762
class Customer < ActiveRecord::Base
1764
:extend => [FindRecentExtension, FindActiveExtension]
1768
Extensions can refer to the internals of the association proxy using these three accessors:
1770
* +proxy_owner+ returns the object that the association is a part of.
1771
* +proxy_reflection+ returns the reflection object that describes the association.
1772
* +proxy_target+ returns the associated object for +belongs_to+ or +has_one+, or the collection of associated objects for +has_many+ or +has_and_belongs_to_many+.
1776
"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213-rails-guides/tickets/11
1778
* February 1, 2009: Added +:autosave+ option "Mike Gunderloy":credits.html#mgunderloy
1779
* September 28, 2008: Corrected +has_many :through+ diagram, added polymorphic diagram, some reorganization by "Mike Gunderloy":credits.html#mgunderloy . First release version.
1780
* September 22, 2008: Added diagrams, misc. cleanup by "Mike Gunderloy":credits.html#mgunderloy (not yet approved for publication)
1781
* September 14, 2008: initial version by "Mike Gunderloy":credits.html#mgunderloy (not yet approved for publication)
added
removed
vendor/rails/railties/guides/source/association_basics.textile
