Hi,
I’m having a problem with the collection append method in a many-to-
many relationship with association object. Basically, I’d like to
connect classes of type Entity through a Context object (which holds a
description of the connection type). Programmatically, it should go
like
experiment = Entity()
observer = Entity()
experiment.connect("has Observer", observer)
print experiment.connected # prints [observer]
However, what happens is that this code results in observer being
printed twice unless some other operation occurred before that
statement. It is not really clear to me why this happens. The code is
as follows:
from sqlalchemy import Column, ForeignKey, String, Integer,
create_engine
from sqlalchemy.orm import relationship, backref, scoped_session,
sessionmaker
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Entity(Base):
__tablename__ = "entities"
id = Column('id', Integer, primary_key=True)
def connect(self, connection_type, connection_object):
"""Connect this entity with connection_object via the
connection_type."""
context = Context(back_referenced=self,
connected=connection_object, connection_type=connection_type)
self.connections.append(context)
@property
def connected(self):
return [c.connected for c in self.connections]
class Context(Base):
__tablename__ = "contexts"
entity_id = Column('entity_id', Integer,
ForeignKey(Entity.id), primary_key=True)
connected_id = Column('connected_id', Integer,
ForeignKey(Entity.id), primary_key=True)
connection_type = Column('connection_type', String(500))
# Each entity can have a context of related entities
back_referenced = relationship(Entity,
backref=backref('connections', cascade="all"),
primaryjoin=entity_id==Entity.id)
connected = relationship(Entity,
backref=backref('back_references', cascade="all"),
primaryjoin=connected_id==Entity.id)
def __repr__(self):
return "Context(entity_id={id!s}, connected_id={cid!s},
connection_type={type})".format(id=self.entity_id,
cid=self.connected_id, type=self.connection_type)
def __str__(self):
return "Context({e} has {t}
{c})".format(e=self.back_referenced, t=self.connection_type,
c=self.connected)
session = scoped_session(sessionmaker(autocommit=True))
engine = create_engine("sqlite://")
session(bind=engine)
Base.metadata.create_all(engine)
e1 = Entity()
e2 = Entity()
session.add(e1)
session.add(e2)
e1.connect("C", e2)
print e1.connected
print "Number of connections to e1", len(e1.connected)
# prints:
# [<__main__.Entity object at 0x101613dd0>, <__main__.Entity object at
0x101613dd0>]
# Number of connections to e1 2
Is there anything I can do to get the correct number of entities back?
Thanks
/rike
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