I'm working on a system that has a Base class called Object. Many
other classes inherit from this one class via joined table inheritance
and it's working great. But now there's a problem: The subclass
"Comment" does reference "Object" twice, once for the inheritance and
once as a ForeignKey to mark which object the comment is about.
Code example:
----------------------------
Base = declarative_base()
class Object(Base):
"Base class for all our Entities"
__tablename__= 'objects'
uuid=Column(String(36),primary_key=True)
discriminator=Column(String(30), nullable=False)
__mapper_args__ = {'polymorphic_on': discriminator,
'polymorphic_identity': 'object'}
def __init__(self,*args,**kwargs):
super(Object,self).__init__(*args,**kwargs)
if not self.uuid:
# create a random uuid
self.uuid=str(uuid4())
def __repr__(self):
return u"<Object('%s')>" % (self.uuid)
class Comment(Object):
"""Allows commenting of any other entity in the server"""
__tablename__= 'comments'
__mapper_args__ = {'polymorphic_identity': 'comment'}
uuid=Column(String(36),ForeignKey
('objects.uuid'),primary_key=True)
text=Column(String(),nullable=False)
author=Column(String(),nullable=False)
date=Column(AutoNowDateTime(),nullable=False)
subject_id=Column(String(36),ForeignKey('objects.uuid'))
def __repr__(self):
return u"<Comment('%s','%s','%s')>" % (self.author,str
(self.date),self.text[:20])
------------------
This leads to this error:
#ArgumentError: Can't determine join between 'objects' and
'comments';
#tables have more than one foreign key constraint relationship between
them.
#Please specify the 'onclause' of this join explicitly.
I tried to add __mapper_args__ to Comment like
"inherit_condition=Comment.c.uuid==Object.c.uuid"
but while I can get it to initialize, it won't let me query giving me
this error:
AttributeError: 'PropertyLoader' object has no attribute 'strategy'
How can I allow more than one reference between those Classes?
Thanks
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