On Mar 12, 2013, at 4:50 PM, chris.r.mcgu...@gmail.com wrote:
> Hi all,
>
> What is the best way to avoid querying the base table in an joined table
> inheritance? For instance with a class setup like:
>
> class A(Base):
> __tablename__ = 'table_a'
> id = Column(Integer, primary_key=True)
> name = Column(String(50))
>
> class B(A):
> __tablename__ = 'table_b'
> id = Column(Integer, primary_key=True)
> email = Column('mail_addr', String(50))
>
> user_id = Column(Integer, ForeignKey('table_a.id'))
>
> If I were to query against B `session.query(B)`, the generated query would
> join on results in something like : select ... FROM table_a JOIN table_b ON
> table_a.id = table_b.user_id. Which has been fine up until now. But now I
> need need to perform a query solely against table_b. Is there a way I can
> specify in the query not to perform the join to table_a?
>
> I know from the docs that I can query the table_b table directly via
> session.query(B.__table__), however the results of that query end up in
> NamedTuples, whereas I'd like the results to be instances of class B.
there's no straightforward way to get a full B without loading the "A" row
because "A" is where the primary key of your composed "B" is, and all objects
loaded must have their identity present. There are very hacky ways to make it
happen, like constructing a "B" for yourself using some unsavory APIs and
adding it in, but you'd lose eager loading capability and querying would be
awkward.
If your application has lots of cases where it wants to load "B"-like objects
without their "A", that suggests that perhaps you want to build a class of
objects where "A" is not part of the hierarchy, but is instead loaded using a
one-to-one relationship() instead. Your "user_id" foreign key column already
looks more like that, than an inheritance situation.
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