On Nov 11, 2011, at 3:46 AM, Vlad K. wrote:
>
> Hi,
>
>
> I have two models, A and B. Model B contains two foreign keys into table A,
> because it is a "comparator" model that describes certain logical interaction
> between two A models. However, I want model B to contain a relationship to
> both so I can access them through the model B instance:
>
>
> class ModelB(Base):
> __tablename__ = ...
>
> id_b = ... # primary
> some_comparison_data = ...
>
> main_model_id = Column(Integer, ForeignKey("models_a.id_a",
> ondelete="cascade", onupdate="cascade"))
> duplicate_model_id = Column(Integer, ForeignKey("models_a.id_a",
> ondelete="cascade", onupdate="cascade"))
>
> main_model = relationship("ModelA", foreign_keys=[ ??? ], lazy="joined")
> duplicate_model = relationship("ModelA", foreign_keys=[ ??? ],
> lazy="joined")
>
>
>
> ModelA has no keys back to B, this is basically a "one-to-two" relationship
> from B to A.
I would classify this as two distinct "many to one" relationships from B to A.
>
> Now as you can see I don't know what to specify for foreign keys.
You don't need to specify foreign_keys here as the Column objects on ModelB
already have valid ForeignKey objects back to ModelA. You do however need to
specify "primaryjoin" for each relationship() here, as it is otherwise
ambiguous how a join from ModelB to ModelA should proceed:
main_model = relationship("ModelA",
primaryjoin="ModelA.id_a==ModelB.main_model_id")
duplicate_model = relationship("ModelA",
primaryjoin="ModelA.id_a==ModelB.duplicate_model_id")
> I tried with foreign_keys=[main_model_id] and
> foreign_keys=[duplicate_model_id] but it complains it couldn't determine
> join condition and that I should use primaryjoin. Evenso I wouldn't know how
> to use it because I don't know what to specify: class variable? a string?
> The examples in the docs are not for declarative...
I'm always looking to improve the documentation. In this case, all examples in
the "relationship" documentation now use declarative for most examples,
including "primaryjoin":
http://www.sqlalchemy.org/docs/orm/relationships.html#specifying-alternate-join-conditions-to-relationship
It's also in the declarative extension docs:
http://www.sqlalchemy.org/docs/orm/extensions/declarative.html#configuring-relationships
Perhaps you were looking at older documentation ?
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