That is what I initially had,
I get the error
sqlalchemy.exc.InvalidRequestError: Select statement 'SELECT type.type
FROM type, thing
WHERE type.id = thing.type_id' returned no FROM clauses due to
auto-correlation; specify correlate(<tables>) to control correlation
manually.
On Thursday, February 9, 2017 at 10:02:40 AM UTC-6, Mike Bayer wrote:
>
>
>
> On 02/09/2017 10:14 AM, Shane Carey wrote:
> > Hi, another question.
> >
> > I set my discriminator on the surrogate primary key of its table.
> > However, when I query for all of a certain discriminator, I get an error
> > 'Multiple rows returned for subquery'.
> >
> > Here is my canonical example
> >
> > from sqlalchemy import *
> > from sqlalchemy import select, and_, event, inspect
> > from sqlalchemy.orm import *
> > from sqlalchemy.ext.declarative import *
> >
> > Base = declarative_base()
> >
> > class Type(Base):
> > __tablename__ = 'type'
> >
> > id = Column(Integer, primary_key=True)
> > type = Column(String(8), unique=True)
> >
> > class Thing(Base):
> > __tablename__ = 'thing'
> >
> > id = Column(Integer, primary_key=True)
> > type_id = Column(Integer, ForeignKey(Type.id), nullable=False)
> >
> > type = relationship(Type)
> >
> > __mapper_args__ = { 'polymorphic_on': select([Type.type]).where(Type.id
> > == type_id).correlate(Type).as_scalar(), 'with_polymorphic': '*' }
> >
> > @event.listens_for(Thing, 'init', propagate=True)
> > def set_identity(instance, *args, **kwargs):
> > instance.type_id = select([Type.id]).where(Type.type ==
> > object_mapper(instance).polymorphic_identity)
> >
> > class Stuff(Thing):
> > stuff = Column(String(8))
> > __mapper_args__ = { 'polymorphic_identity': 'stuff' }
> >
> > class Junk(Thing):
> > junk = Column(String(8))
> > __mapper_args__ = { 'polymorphic_identity': 'junk' }
> >
> > if __name__ == '__main__':
> > e = create_engine('mysql+pymysql://user:password@localhost/test',
> echo=True)
> >
> > Base.metadata.drop_all(e)
> > Base.metadata.create_all(e)
> >
> > s = Session(e)
> >
> > s.add_all([Type(type='stuff'), Type(type='junk')])
> > for i in range(10):
> > s.add(Stuff(stuff='stuff_{}'.format(i)))
> > s.add(Junk(junk='junk_{}'.format(i)))
> >
> > s.commit()
> >
> > res = s.query(Thing).join(Type).filter(Type.type == 'stuff').all()
> >
> > #res = s.query(Stuff).all() Also fails
> >
> > At first I was receiving the error 'No FROM table specified due to
> > autocorrelate', but I was able to solve that with 'polymorphic_on':
> > select([Type.type]).where(Type.id ==
> type_id).correlate(Type).as_scalar()
> > which I still do not fully understand. The exact error I get is
>
> I think you want to correlate(Thing) there. correlate(X) means you
> don't want "X" in the FROM list in the subquery. But also the
> correlation should be automatic here so you wouldn't need to refer to
> "Thing" in the class def (if you did, you'd need to turn __mapper_args__
> into a @declared_attr).
>
> that is:
>
> "polymorphic_on": select([Type.type]).where(Type.id ==
> type_id).as_scalar()
>
> should work
>
>
>
> >
> > sqlalchemy.exc.InternalError: (pymysql.err.InternalError) (1242,
> > 'Subquery returns more than 1 row') [SQL: 'SELECT thing.id AS thing_id,
> > thing.type_id AS thing_type_id, thing.text AS thing_text, (SELECT
> > type.type \nFROM type, thing \nWHERE type.id = thing.type_id) AS
> > _sa_polymorphic_on, thing.stuff AS thing_stuff, thing.junk AS thing_junk
> > \nFROM thing']
> >
> > it seems like the SQL it should generate is
> >
> > SELECT thing.id AS thing_id,
> > thing.type_id AS thing_type_id,
> > thing.text AS thing_text,
> > (
> > SELECT type
> > FROM type # this was thing before
> > WHERE type.id = thing.type_id
> > ) AS _sa_polymorphic_on,
> > thing.stuff AS thing_stuff,
> > thing.junk AS thing_junk
> > FROM thing
> > INNER JOIN type ON type.id = thing.type_id
> > WHERE type.type = 'junk';
> >
> > but better again would be, this, right?
> >
> > SELECT thing.id AS thing_id,
> > thing.type_id AS thing_type_id,
> > thing.text AS thing_text,
> > type.type AS _sa_polymorphic_on,
> > thing.stuff AS thing_stuff,
> > thing.junk AS thing_junk
> > FROM thing
> > INNER JOIN type ON type.id = thing.type_id
> > WHERE type.type = 'junk';
> >
> > so I am not sure what is going wrong or how I can generate the correct
> SQL,
> > or if possible generate a join rather than a subquery.
> >
> > Thanks for you help and patience
> >
> > --
> > SQLAlchemy -
> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
> >
> > To post example code, please provide an MCVE: Minimal, Complete, and
> > Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> > description.
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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