On Nov 12, 2008, at 9:42 AM, tante wrote:
>
> 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?
strings aren't recognized for "inherit_condition", so do the dict lke
__mapper_args__ = {'inherit_condition':uuid==Object.uuid}, and place
__mapper_args__ in Comment after you define the "uuid" coumn.
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