Creating a container relation in declarative SQLAlchemy

Question

The My Python / SQLAlchemy application manages a collection of nodes, all derived from the Node base class. I am using SQLAlchemy polymorphism functions to manage nodes in a SQLite3 table. Here's the definition of a Node base 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, for 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, which is an ordered container with zero or more nodes. When I load a ListNode, I want it to have its identifier and title (from the Node base class) along with a set of its contained (child) nodes. A Node can appear in multiple ListNode lists, so it is not a proper hierarchy. I would create them in the following 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)

Node - , , . ( , ).

, , , , :

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")

: ListNode items SQLAlchemy , "list" "_sa_instance_state". , ,

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Answer 1

" ":

nodes_list_nodes = Table(
    'nodes_list_nodes', metadata,
    Column('parent_id', None, ForeignKey('nodes_list.id'), nullable=False),
    Column('child_id', None, ForeignKey(Node.id), nullable=False),
    PrimaryKeyConstraint('parent_id', 'child_id'),
)

class ListNode(Node):
    __mapper_args__ = {'polymorphic_identity': 'list'}
    __tablename__ = 'nodes_list'
    id = Column(None, ForeignKey('nodes.id'), primary_key=True)
    items = relation(Node, secondary=nodes_list_nodes)

: association_proxy:

from sqlalchemy.orm.collections import InstrumentedList
from sqlalchemy.ext.associationproxy import association_proxy


class ListNodeAssociation(Base):
    __tablename__ = 'nodes_list_nodes'
    parent_id = Column(None, ForeignKey('nodes_list.id'), primary_key=True)
    child_id = Column(None, ForeignKey(Node.id), primary_key=True)
    order = Column(Integer, nullable=False, default=0)
    child = relation(Node)
    __table_args__ = (
        PrimaryKeyConstraint('parent_id', 'child_id'),
        {},
    )


class OrderedList(InstrumentedList):

    def append(self, item):
        if self:
            item.order = self[-1].order+1
        else:
            item.order = 1
        InstrumentedList.append(self, item)


class ListNode(Node):
    __mapper_args__ = {'polymorphic_identity': 'list'}
    __tablename__ = 'nodes_list'
    id = Column(None, ForeignKey('nodes.id'), primary_key=True)
    _items = relation(ListNodeAssociation,
                      order_by=ListNodeAssociation.order,
                      collection_class=OrderedList,
                      cascade='all, delete-orphan')
    items = association_proxy(
                '_items', 'child',
                creator=lambda item: ListNodeAssociation(child=item))