On Fri, Mar 29, 2019 at 12:17 PM 'Neil Youngman' via sqlalchemy
<[email protected]> wrote:
>
> I'm trying to do a select of columns from a join and I simply can't get it to
> work. I have tried various different ways to specify the join, with and
> without a select_from() and I can't find a combination that works.
>
> the tables look like
>
> class Dealer(Base):
> __tablename__ = 'dealers'
>
> id = Column(String(64), primary_key=True, nullable=False)
> name =Column(String(100), nullable=False)
> phone_number = Column(String(64), nullable=False)
>
> def __repr__(self):
> return "<Dealer(supplying_dealer='%s', registration='%s',
> servicing_dealer='%s')>" % (
> self.supplying_dealer, self.registration,
> self.servicing_dealer)
>
>
> class Vehicle(Base):
> __tablename__ = 'vehicles'
>
> registration = Column(String(10), primary_key=True, nullable=False)
> supplying_dealer = Column(String(64), ForeignKey(u'dealers.id'),
> nullable=False)
> servicing_dealer = Column(String(64), ForeignKey(u'dealers.id'),
> nullable=False)
>
> def __repr__(self):
> return "<Vehicle(registration='%s', supplying_dealer='%s',
> servicing_dealer='%s')>" % (
> self.registration, self.supplying_dealer, self.servicing_dealer)
>
> and I have tried lots of variations on:
>
> for supplier, phone, registration in
> session.query(Dealer.name,Dealer.phone_number,Vehicle.registration).select_from(Vehicle).join('supplying_dealer'):
> print( supplier, phone, registration )
>
> I haven't found a combination that works. Am I missing the obvious?
You haven't created a relationship() between Vehicle and Dealer, which
reduces your options a little bit. This should work:
session.query(Vehicle).join(Dealer, Vehicle.supplying_dealer == Dealer.id)
ie. you need to be explicit about the class you are joining to, and
the join condition. The join condition is necessary because you have 2
foreign keys between Vehicle and Dealer.
You might want to consider creating relationships between the two
classes, something like this:
class Vehicle(Base):
__tablename__ = 'vehicles'
registration = Column(String(10), primary_key=True, nullable=False)
# note that I've renamed these columns
supplying_dealer_id = Column(String(64),
ForeignKey(u'dealers.id'), nullable=False)
servicing_dealer_id = Column(String(64),
ForeignKey(u'dealers.id'), nullable=False)
supplying_dealer = relationship(Dealer, supplying_dealer_id == Dealer.id)
servicing_dealer = relationship(Dealer, servicing_dealer_id == Dealer.id)
With those relationships in place, either of these joins should work:
session.query(Vehicle).join("supplying_dealer")
session.query(Vehicle).join(Vehicle.supplying_dealer)
Hope that helps,
Simon
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