I'd discourage using metaclasses as they are more difficult to use in cases
like these than mixins, which can accomplish the vast majority of use cases.
I can't help with your metaclass case since there is no code here to work with,
but in general a dynamic __mapper_args__, whether using a mixin or a base
class, can be done like this:
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.ext.declarative import declarative_base, declared_attr
Base = declarative_base()
class MyMixin(object):
@declared_attr
def __mapper_args__(cls):
base_id = cls.__bases__[1].id # or "A.id", or whatever you need to do
# to get to the correct superclass
this_id = cls.id
return {"inherit_condition": base_id == this_id}
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
class B(MyMixin, A):
__tablename__ = 'b'
id = Column(Integer, ForeignKey('a.id'), primary_key=True)
On Jul 3, 2013, at 6:42 AM, Alexey Vihorev <[email protected]> wrote:
> Hi! Got this setup:
>
>
> class Document(Base):
> __tablename__ = 'document'
> discriminator = Column('type', String(30))
> __mapper_args__ = {'polymorphic_identity':'document',
> 'polymorphic_on': discriminator}
> id = Column(Integer, primary_key=True)
> number = String(20)
>
> class Invoice(Document):
> __tablename__ = 'invoice'
> __mapper_args__ = {'polymorphic_identity':'invoice'}
> id = Column(Integer, ForeignKey('document.id'), primary_key=True)
> customer = String(100)
> linked_document_id = Column(Integer, ForeignKey('document.id'))
>
>
> And, predictably, I get “sqlalchemy.exc.AmbiguousForeignKeysError: Can't
> determine join between 'document' and 'invoice'; tables have more than one
> foreign key constraint relationship between them. Please specify the
> 'onclause' of this join explicitly.” As I understand, augmenting Invoice’s
> __mapper_args__ with 'inherit_condition ': (id == Document.id) will deal with
> the problem. However, in my case I cannot call(id == Document.id), because it
> all is going on inside a metaclass I’ve devised for reasons beyond this topic
> and the class being created (Invoice) does not yet exist at the time of
> setting __mapper_args__ , so accessing this ID class member is problematic.
> So the question is this: can I set 'inherit_condition' by using some other
> way, like string form or something? Thanks!
>
>
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