Ah, so, thanks. My logic was that I could specify which foreign_key to
use for the relationship, which is basically a subset of primaryjoin
condition, but in my opinion "cleaner". So, that wouldn't work? I must
use always use primaryjoin?
I was looking at few paragraphs below, under Multiple Relationships
against the same Parent/Child, which is basically my situation, and
shows a mapper based solution.
http://www.sqlalchemy.org/docs/orm/relationships.html#multiple-relationships-against-the-same-parent-child
Thanks for help!
.oO V Oo.
On 11/11/2011 07:32 PM, Michael Bayer wrote:
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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