Thanks for your reply!
> you could probably insert another class as the parent of A1, with no
polymorphic identity.
This is what I would like to avoid. Otherwise I would need to write some
metaclass machinery to inject base class for all hundred models I have.
Originally I am going to add this __mapper_args__ via mixin with
declared_attr.
пʼятниця, 21 червня 2019 р. 15:08:47 UTC+3 користувач Simon King написав:
>
> From a quick test, this appears to be special behaviour for the base
> class in an inheritance hierarchy. If you extend your test like this:
>
> from sqlalchemy import Column, Integer, String
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.orm import Query, configure_mappers
>
> Base = declarative_base()
>
> class A1(Base):
> __tablename__ = 'a1'
> id = Column(Integer, primary_key=True)
> poly_on = Column(String, nullable=False)
> __mapper_args__ = {
> 'polymorphic_on': poly_on,
> 'polymorphic_identity': 'a1',
> }
>
> class A2(A1):
> __mapper_args__ = {
> 'polymorphic_identity': 'a2',
> }
>
> class A2a(A2):
> __mapper_args__ = {
> 'polymorphic_identity': 'a2a',
> }
>
> class A2b(A2):
> __mapper_args__ = {
> 'polymorphic_identity': 'a2b',
> }
>
> if __name__ == '__main__':
> for cls in [A1, A2, A2a, A2b]:
> print('#' * 10)
> print(cls)
> print(Query(cls))
>
>
> ...then the output looks like this:
>
> ##########
> <class '__main__.A1'>
> SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> FROM a1
> ##########
> <class '__main__.A2'>
> SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> FROM a1
> WHERE a1.poly_on IN (:poly_on_1, :poly_on_2, :poly_on_3)
> ##########
> <class '__main__.A2a'>
> SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> FROM a1
> WHERE a1.poly_on IN (:poly_on_1)
> ##########
> <class '__main__.A2b'>
> SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> FROM a1
> WHERE a1.poly_on IN (:poly_on_1)
>
>
> In other words, if you query for A1, you will get every row from the
> table, even if the polymorphic identity doesn't match any subclasses.
> This makes sense if you think of every row in the table as being some
> kind of (possibly specialized) A1.
>
> However, if you query for A2, you will only get rows that are
> specifically A2, A2a or A2b.
>
> If you don't want this behaviour, you could probably insert another
> class as the parent of A1, with no polymorphic identity. At that
> point, I think queries for A1 would include "WHERE poly_on IN ('a1',
> 'a2')".
>
> Hope that helps,
>
> Simon
>
>
> On Fri, Jun 21, 2019 at 12:22 PM Dmytro Starosud <d.st...@gmail.com
> <javascript:>> wrote:
> >
> > Base class A1 contains polymorphic_identity (along with polymorphic_on),
> but Query(A1)doesn't produce where clause, whereas Query(A2) (where A2 is
> subclass of A1 with its own polymorphic_identity) does.
> >
> > Tried looking in docs with no success. I think I am just missing
> something.
> >
> > from sqlalchemy import Column, Integer, String
> > from sqlalchemy.ext.declarative import declarative_base
> > from sqlalchemy.orm import Query, configure_mappers
> >
> > Base = declarative_base()
> >
> >
> > class A1(Base):
> > __tablename__ = 'a1'
> > id = Column(Integer, primary_key=True)
> > poly_on = Column(String, nullable=False)
> > __mapper_args__ = {
> > 'polymorphic_on': poly_on,
> > 'polymorphic_identity': 'a1',
> > }
> >
> >
> > class A2(A1):
> > __mapper_args__ = {
> > 'polymorphic_identity': 'a2',
> > }
> >
> >
> > configure_mappers()
> >
> > print(Query(A1))
> > # SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> > # FROM a1
> >
> > print(Query(A2))
> > # SELECT a1.id AS a1_id, a1.poly_on AS a1_poly_on
> > # FROM a1
> > # WHERE a1.poly_on IN (:poly_on_1)
> >
> > I would expect WHERE clauses in both cases.
> >
> >
> > Originally posted here.
> >
> > --
> > SQLAlchemy -
> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
> >
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--
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The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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