On Jun 10, 2013, at 2:00 PM, Daniel Grace <[email protected]> wrote:
> I'm converting some existing code from reflecting tables in the database to
> actually defining all of the columns. One of my primary reasons for doing
> this is being able to apply mixins and such to table definitions -- for
> instance, there's numerous lookup tables which all have "id" and "name"
> columns.
>
> My problems arise when trying to declare an order_by that references columns
> defined in a superclass -- e.g: wanting to do something like this:
>
> class LookupTableMixin():
> def __str__(self):
> return self.name
>
> class LookupTable(Model, LookupTableMixin):
> __abstract__ = True
> id = Column(INTEGER(), primary_key=True)
> name = Column(TEXT(), nullable=False)
>
> class SortedLookupTable(LookupTable):
> __abstract__ = True
> __mapper_args__ = { 'order_by': [ name ] }
>
>
> I'm trying to determine the correct way to do this:
> 4. Simply redefining 'id' and 'name' (e.g. copy-pasting the column
> definitions from LookupTable to SortedLookupTable) ultimately causes the
> following:
>
> CompileError("Cannot compile Column object until its 'name' is assigned.",)
>
> Oddly enough, this only happens if __mapper_args__ is also present (and
> ordering by the column)
>
> 5. __mapper_args__ does not appear to work with declared_attr, such that:
>
> @declared_attr
> def __mapper_args__(cls):
> return { 'order_by': [ cls.name ] }
>
> causes the same error as above.
Using 0.8 you can pull the actual copied column "name" from the __table__:
class SortedLookupTable(LookupTable):
__abstract__ = True
@declared_attr
def __mapper_args__(cls):
return {'order_by': [cls.__table__.c.name]}
"cls.name" is the un-copied Column object still associated with the LookupTable
abstract class, ready for use on other mapped classes. This pattern is
discussed in this section:
http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/declarative.html#resolving-column-conflicts
.
Also as far as the "order_by" mapper argument, there's no plans to remove it,
but it's a legacy argument that doesn't work well/at all with many queries
(such as queries against multiple entities, queries against individual columns).
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