On 6/29/15 8:47 AM, Burak Arslan wrote:
hello,
here's a test case:
https://gist.github.com/plq/5ed0c135222ea76d77fc (also see below)
is it possible to preserve the polymorphic_identity values of
subclasses that contain changes to non-sqla parts of a class?
I understand that the reason for this is D2.__mapper__.class_ != D2.
Is there a way to clone the class mapper and change just the mapped class?
you can do this:
class D2(D):
foo = 'bar'
__mapper_args__ = {
'polymorphic_identity': 2,
}
though you'll get a warning.
SQLAlchemy expects that you will build up the functionality of your
class *underneath* the mapping, that is, you can use mixins and
__abstract__ to apply behaviors to your class, but the descendant-most
class is the thing you're persisting. if you say session.add(D()) and
session.add(D2()), the ORM will give you indeterminate results; if an
identity is already present and it's a D2, you'll get a D2 back from a
result set, otherwise you'll get a D. The design of mappings
discourages designs that would produce this issue.
best,
burak
ps: the test case:
from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class C(Base):
__tablename__ = 'C'
id = Column(Integer, primary_key=True)
t = Column(Integer)
__mapper_args__ = {
'polymorphic_on': t,
'polymorphic_identity': 1,
}
assert C().t == 1
class D(C):
__mapper_args__ = {
'polymorphic_identity': 2,
}
assert D().t == 2
class D2(D):
foo = 'bar'
assert D2().t == 2
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