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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>
> --
> 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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