Hi. I am using sqlalchemy with a database that has two tables that
each have foreign keys on the other.
The situation this is modeling is a tree where the levels alternate
between two different types of node. Each Node1 has a parent of type
Node2 (or null for the root) and each Node2 has a parent of type
Node1.
(These aren't the actual names but I am trying to simplify it down for
exposition).
Using ext.declarative I have this (removing the irrelevant parts):
Base = declarative_base(metadata=metadata)
class Node1(Base):
__tablename__ = 'node1'
id = Column('id', Integer, primary_key=True)
parent_node2_id = Column('parent_node2_id', Integer, ForeignKey('node2.id'))
class Node2(Base):
__tablename__ = 'node2'
id = Column('id', Integer, primary_key=True)
parent_node1_id = Column('parent_node1_id', Integer, ForeignKey('node1.id'))
Node2.parent_node1 = relation(Node1, primaryjoin = Node1.id ==
Node2.parent_node1_id, backref = 'children')
Node1.parent_node2 = relation(Node2, primaryjoin = Node2.id ==
Node1.parent_node2_id, backref = 'children')
This fails (as expected) because of the circular dependency. Before I
switched to declarative, I had solved this by using a
ForeignKeyConstraint with use_alter.
I can't see how to do that with declarative. I tried changing Node2 to:
class Node2(Base):
__tablename__ = 'node2'
id = Column('id', Integer, primary_key=True)
parent_node1_id = Column('parent_node1_id', Integer)
fk = ForeignKeyConstraint(['parent_node1_id'], ['node1.guid'],
use_alter = True, name='parent_key')
SA now complains that my Node2.parent_node1 relation doesn't have a
foreign key, so it looks like the constraint didn't take.
How do I make this work?
Also, I think that I have to put the relations outside of the class
definitions (as above) because the primaryjoin arguments references
the class itself. Is there a way around this?
Thanks
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