it doesn't produce as efficient of a SQL query, and requires that you set
"Type" manually on the object, but you can use a correlated subquery using this
form:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Type(Base):
__tablename__ = 'type'
id = Column(Integer, primary_key=True)
name = Column(String)
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
type_id = Column(Integer, ForeignKey('type.id'), nullable=False)
type = relationship("Type")
type_name = column_property(select([Type.name]).where(Type.id ==
type_id).as_scalar())
__mapper_args__ = {'polymorphic_identity': 'a', 'polymorphic_on': type_name}
class SubA(A):
__tablename__ = 'sub_a'
id = Column(Integer, ForeignKey('a.id'), primary_key=True)
__mapper_args__ = {'polymorphic_identity': 'sub_a'}
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)
ta, tsuba = Type(name='a'), Type(name='sub_a')
s.add_all([ta, tsuba])
s1 = SubA(type=tsuba)
s2 = A(type=ta)
s.add_all([s1, s2])
s.commit()
s.close()
print s.query(A).filter_by(id=1).one()
the other way to go is just to have polymorphic_on be "type_id" and use the
integer identifiers as the polymorphic identities.
On Jan 28, 2013, at 7:52 PM, Ben Hitz wrote:
> Can I use a field from a related table as the polymorphic descriminator?
>
> This code no works:
> http://pastebin.com/LtRC2tSR
>
> Schematically:
> class Reference:
> reference_id
> reference_name
>
> class Parent:
> parent_id
> reference_id(ForeignKey='reference.reference_id')
>
> reference = relationship(Reference, useList=False)
>
> __mapper_args__ {
> polymorphic_on: reference.reference_name
> }
>
> class Child
> child_id(ForeignKey='parent.parent_id')
> child_property
>
> __mapper_args__ {
> polymorphic_identity: "some value of reference.name"
> }
>
> I also tried an association_proxy but I get sqlalchemy.exc.ArgumentError:
> Only direct column-mapped property or SQL expression can be passed for
> polymorphic_on
>
> I guess there is someway to extract the SQL emitted by the relationship and
> use that as the value of polymorphic_on? Or do I have to use something like:
>
> select([Reference.reference_name].join(Parent)
>
> Thanks
> Ben
>
>
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