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        Release: <span class="version-num">0.9.6</span> | Release Date: June 23, 2014

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            <h3><a href="../index.html">SQLAlchemy 0.9 Documentation</a></h3>

 

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        <h3><a href="#">            

                Relationship Configuration

            

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<li><a class="reference internal" href="#non-relational-comparisons-materialized-path">Non-relational Comparisons / Materialized Path</a></li>

 

 

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order to load or represent this relationship. That is, it&#8217;s used in

<div class="section" id="non-relational-comparisons-materialized-path">

<h3>Non-relational Comparisons / Materialized Path<a class="headerlink" href="#non-relational-comparisons-materialized-path" title="Permalink to this headline">¶</a></h3>

<div class="admonition warning">

<p class="first admonition-title">Warning</p>

<p class="last">this section details an experimental feature.</p>

</div>

<p>Using custom expressions means we can produce unorthodox join conditions that

don&#8217;t obey the usual primary/foreign key model.  One such example is the

materialized path pattern, where we compare strings for overlapping path tokens

in order to produce a tree structure.</p>

<p>Through careful use of <a class="reference internal" href="#sqlalchemy.orm.foreign" title="sqlalchemy.orm.foreign"><tt class="xref py py-func docutils literal"><span class="pre">foreign()</span></tt></a> and <a class="reference internal" href="#sqlalchemy.orm.remote" title="sqlalchemy.orm.remote"><tt class="xref py py-func docutils literal"><span class="pre">remote()</span></tt></a>, we can build

a relationship that effectively produces a rudimentary materialized path

system.   Essentially, when <a class="reference internal" href="#sqlalchemy.orm.foreign" title="sqlalchemy.orm.foreign"><tt class="xref py py-func docutils literal"><span class="pre">foreign()</span></tt></a> and <a class="reference internal" href="#sqlalchemy.orm.remote" title="sqlalchemy.orm.remote"><tt class="xref py py-func docutils literal"><span class="pre">remote()</span></tt></a> are

on the <em>same</em> side of the comparison expression, the relationship is considered

to be &#8220;one to many&#8221;; when they are on <em>different</em> sides, the relationship

is considered to be &#8220;many to one&#8221;.   For the comparison we&#8217;ll use here,

we&#8217;ll be dealing with collections so we keep things configured as &#8220;one to many&#8221;:</p>

<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Element</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>

    <span class="n">__tablename__</span> <span class="o">=</span> <span class="s">&#39;element&#39;</span>

 

    <span class="n">path</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>

 

    <span class="n">descendants</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">&#39;Element&#39;</span><span class="p">,</span>

                           <span class="n">primaryjoin</span><span class="o">=</span>

                                <span class="n">remote</span><span class="p">(</span><span class="n">foreign</span><span class="p">(</span><span class="n">path</span><span class="p">))</span><span class="o">.</span><span class="n">like</span><span class="p">(</span>

                                        <span class="n">path</span><span class="o">.</span><span class="n">concat</span><span class="p">(</span><span class="s">&#39;/%&#39;</span><span class="p">)),</span>

                           <span class="n">viewonly</span><span class="o">=</span><span class="bp">True</span><span class="p">,</span>

                           <span class="n">order_by</span><span class="o">=</span><span class="n">path</span><span class="p">)</span></pre></div>

</div>

<p>Above, if given an <tt class="docutils literal"><span class="pre">Element</span></tt> object with a path attribute of <tt class="docutils literal"><span class="pre">&quot;/foo/bar2&quot;</span></tt>,

we seek for a load of <tt class="docutils literal"><span class="pre">Element.descendants</span></tt> to look like:</p>

<div class="highlight-python"><pre>SELECT element.path AS element_path

FROM element

WHERE element.path LIKE ('/foo/bar2' || '/%') ORDER BY element.path</pre>

</div>

<div class="versionadded">

<p><span>New in version 0.9.5: </span>Support has been added to allow a single-column

comparison to itself within a primaryjoin condition, as well as for

primaryjoin conditions that use <tt class="xref py py-meth docutils literal"><span class="pre">Operators.like()</span></tt> as the comparison

operator.</p>

</div>

</div>

<p><a class="reference internal" href="#self-referential"><em>Adjacency List Relationships</em></a> - in-depth explanation of how