On Aug 5, 2010, at 11:06 PM, anthony wrote:
> Hi,
>
> First message here in the sqlalchemy mailing list. I was working with
> a previous version of sqlalchemy (0.6beta1) and while updating
> sqlalchemy, ran into a problem with polymorphic classes. Basically
> we're using a single table inheritance scheme and we want the "value"
> column to have different types. In the case of the example, the parent
> is A, and I wanted child B and C to interpret the "value" column of
> our table differently depending on the type. This worked in the
> previous version of but no longer works :(.
>
> class A(Base):
> id = Column(Integer)
> type = Column(Integer)
> __mapper_args__ = {'polymorphic_on': type}
>
> class B(A):
> __mapper_args = {'polymorphic_identity': 1}
> value = Column(Integer)
>
> class C(A):
> __mapper_args = {'polymorphic_identity': 2}
> value = Column(Boolean)
it works fine as expected with joined inheritance. With single table, it
doesn't make any sense as "value" is a column on your table. Is the table
column int or boolean ? I imagine you're hitting upon some sanity assertions
that weren't in the beta version (always helpful if these details can be
included...what style of inheritance, what does "no longer works :(" mean, etc.)
If you want the subclasses to interpret the common "value" column, which here
should probably be defined on A as the actual database column type, in
different ways, use descriptors:
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
type = Column(Integer)
_value = Column('value', String)
__mapper_args__ = {'polymorphic_on': type}
class B(A):
__mapper_args = {'polymorphic_identity': 1}
@property
def value(self):
return int(self._value)
class C(A):
__mapper_args = {'polymorphic_identity': 2}
@property
def value(self):
return bool(self._value)
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