Hi! I've posted this question over on Stack Overflow -- thought
perhaps the mailing list might be a better resource for help. (If
you're a Stack Overflow user and want the points, here's the link:
http://stackoverflow.com/questions/1791713/creating-container-relationship-in-declarative-sqlalchemy
)
My Python / SQLAlchemy application manages a set of nodes, all derived
from a base class Node. I'm using SQLAlchemy's polymorphism features
to manage the nodes in a SQLite3 table. Here's the definition of the
base Node class:
class Node(db.Base):
__tablename__ = 'nodes'
id = Column(Integer, primary_key=True)
node_type = Column(String(40))
title = Column(UnicodeText)
__mapper_args__ = {'polymorphic_on': node_type}
and, as an example, one of the derived classes, NoteNode:
class NoteNode(Node):
__mapper_args__ = {'polymorphic_identity': 'note'}
__tablename__ = 'nodes_note'
id = Column(None,ForeignKey('nodes.id'),primary_key=True)
content_type = Column(String)
content = Column(UnicodeText)
Now I need a new kind of node, ListNode, that is an ordered container
of zero or more Nodes. When I load a ListNode, I want it to have its
ID and title (from the base Node class) along with a collection of its
contained (child) nodes. A Node may appear in more than one ListNode,
so it's not a proper hierarchy. I would create them along these lines:
note1 = NoteNode(title=u"Note 1", content_type="text/text",
content=u"I am note #1")
session.add(note1)
note2 = NoteNode(title=u"Note 2", content_type="text/text",
content=u"I am note #2")
session.add(note2)
list1 = ListNode(title=u"My List")
list1.items = [note1,note2]
session.add(list1)
The list of children should only consist of Node objects -- that is,
all I need is their base class stuff. They shouldn't be fully realized
into the specialized classes (so I don't get the whole graph at once,
among other reasons).
I started along the following lines, cobbling together bits and pieces
I found in various places without a complete understanding of what was
going on, so this may not make much sense:
class ListNode(Node):
__mapper_args__ = {'polymorphic_identity': 'list',
'inherit_condition':id==Node.id}
__tablename__ = 'nodes_list_contents'
id = Column(None, ForeignKey('nodes.id'), primary_key=True)
item_id = Column(None, ForeignKey('nodes.id'), primary_key=True)
items = relation(Node, primaryjoin="Node.id==ListNode.item_id")
This approach fails in several ways: it doesn't appear to allow an
empty ListNode, and setting the items attribute to a list results in
SQLAlchemy complaining that 'list' object has no attribute
'_sa_instance_state'. Not surprisingly, hours of random mutations on
this theme haven't given any good results,
I have limited experience in SQLAlchemy but really want to get this
working soon. I'd very much appreciate any advice or direction you can
offer. Thanks in advance!
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