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"""Illustrate a 'dict of sets of integers' model.
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This is a three table model which represents a parent table referencing a
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dictionary of string keys and sets as values, where each set stores a
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collection of integers. The association proxy extension is used to hide the
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details of this persistence. The dictionary also generates new collections
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upon access of a non-existent key, in the same manner as Python's
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"collections.defaultdict" object.
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from sqlalchemy import String, Integer, Column, create_engine, ForeignKey
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from sqlalchemy.orm import relationship, Session
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from sqlalchemy.orm.collections import MappedCollection
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from sqlalchemy.ext.declarative import declarative_base
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from sqlalchemy.ext.associationproxy import association_proxy
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id = Column(Integer, primary_key=True)
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Base = declarative_base(cls=Base)
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class GenDefaultCollection(MappedCollection):
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def __missing__(self, key):
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self[key] = b = B(key)
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associations = relationship("B",
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collection_class=lambda: GenDefaultCollection(operator.attrgetter("key"))
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collections = association_proxy("associations", "values")
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"""Bridge the association from 'associations' over to the 'values'
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association proxy of B.
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a_id = Column(Integer, ForeignKey("a.id"), nullable=False)
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elements = relationship("C", collection_class=set)
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values = association_proxy("elements", "value")
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"""Bridge the association from 'elements' over to the
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'value' element of C."""
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def __init__(self, key, values=None):
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b_id = Column(Integer, ForeignKey("b.id"), nullable=False)
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value = Column(Integer)
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def __init__(self, value):
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if __name__ == '__main__':
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engine = create_engine('sqlite://', echo=True)
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Base.metadata.create_all(engine)
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session = Session(engine)
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# only "A" is referenced explicitly. Using "collections",
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# we deal with a dict of key/sets of integers directly.
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a1 = session.query(A).first()
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print a1.collections["1"]
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a1.collections["1"].add(4)
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a1.collections["2"].update([7, 8, 9])
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print a1.collections["2"]
added
removed
examples/association/dict_of_sets_with_default.py
