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POAttribute as dict of lists using middleman table with SQLAlchemy
 

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    <p>My question is about SQLAlchemy but I'm having troubles explaining it in words so I figured I explain it with a simple example of what I'm trying to achieve:</p> <pre><code>parent = Table('parent', metadata, Column('parent_id', Integer, primary_key=True), Column('name', Unicode), ) parent_child = Table('parent_child', metadata, Column('parent_id', Integer, primary_key=True), Column('child_id', Integer, primary_key=True), Column('number', Integer), ForeignKeyConstraint(['parent_id'], ['parent.parent_id']), ForeignKeyConstraint(['child_id'], ['child.child_id']), ) child = Table('child', metadata, Column('child_id', Integer, primary_key=True), Column('name', Unicode), ) class Parent(object): pass class ParentChild(object): pass class Child(object): pass &gt;&gt;&gt; p = Parent(name=u'A') &gt;&gt;&gt; print p.children {} &gt;&gt;&gt; p.children[0] = Child(name=u'Child A') &gt;&gt;&gt; p.children[10] = Child(name=u'Child B') &gt;&gt;&gt; p.children[10] = Child(name=u'Child C') </code></pre> <p>This code would create 3 rows in parent_child table, column number would be 0 for the first row, and 10 for the second and third row.</p> <pre><code>&gt;&gt;&gt; print p.children {0: [&lt;Child A&gt;], 10: [&lt;Child B&gt;, &lt;Child C&gt;]} &gt;&gt;&gt; print p.children[10][0] &lt;Child B&gt; </code></pre> <p>(I left out all SQLAlchemy session/engine code in the example to make it as clean as possible)</p> <p>I did a try using´</p> <pre><code>collection_class=attribute_mapped_collection('number') </code></pre> <p>on a relation between Parent and ParentChild, but it only gave me one child for each number. Not a dict with lists in it. Any help appreciated!</p> <h3>UPDATED!</h3> <p>Thanks to Denis Otkidach I now has this code, but it still doesn't work.</p> <pre><code>def _create_children(number, child): return ParentChild(parent=None, child=child, number=number) class Parent(object): children = association_proxy('_children', 'child', creator=_create_children) class MyMappedCollection(MappedCollection): def __init__(self): keyfunc = lambda attr_name: operator.attrgetter('number') MappedCollection.__init__(self, keyfunc=keyfunc) @collection.appender @collection.internally_instrumented def set(self, value, _sa_initiator=None): key = self.keyfunc(value) try: self.__getitem__(key).append(value) except KeyError: self.__setitem__(key, [value]) mapper(Parent, parent, properties={ '_children': relation(ParentChild, collection_class=MyMappedCollection), }) </code></pre> <p>To insert an Child seems to work</p> <pre><code>p.children[100] = Child(...) </code></pre> <p>But when I try to print children like this:</p> <pre><code>print p.children </code></pre> <p>I get this error:</p> <pre><code>sqlalchemy.orm.exc.UnmappedInstanceError: Class '__builtin__.list' is not mapped </code></pre>
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