Thank you Simon.
I think you're right. I should better ask another question, instead of
enrolling XY problem here (in short I need to add particular filter to each
model).
But let's wrap up with this one. Did I understand correctly that I *cannot*
have two following things simultaneously:
1. polymorphic_identity on base class;
2. automatic filter on polymorphic_on for base class;
Thanks,
Dmytro
пт, 21 черв. 2019 о 16:42 Simon King <si...@simonking.org.uk> пише:
> The extra base class is only needed once per inheritance hierarchy. Do
> you have a hundred of those? Do you mind if I ask what kind of data
> you are storing, and how you are structuring your class hierarchy?
>
> Thanks,
>
> Simon
>
> On Fri, Jun 21, 2019 at 1:30 PM Dmytro Starosud <d.staro...@gmail.com>
> wrote:
> >
> > 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>
> 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/
> >> >
> >> > 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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> > --
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> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
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> --
> SQLAlchemy -
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>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
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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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