Can't define __table_args__ on a child-class in a single-table inheritance setup

[Andrew Pashkin]

I've found that if __table_args__ is present on a child class in a single-table inheritance setup, SQLAlchemy throws sqlalchemy.exc.ArgumentError: Can't place __table_args__ on an inherited class with no table. At the same time it is possible to define a column with index=True on a child class, which alters the parent table the same way as __table_args__.

Here is my setup:

import sqlalchemy as sa

import sqlalchemy.ext.declarative

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):

    __tablename__ = 'a'

    id = sa.Column(sa.Integer, primary_key=True)

    type = sa.Column(sa.Text, nullable=False)

    __mapper_args__ = {

        'polymorphic_on': type,

    }

class B(A):

    b = sa.Column(sa.Integer, index=True)

    __mapper_args__ = {

        'polymorphic_identity': 'b',

    }

class C(A):

    c = sa.Column(sa.Integer)

    __mapper_args__ = {

        'polymorphic_identity': 'c',

    }

    __table_args__ = (

        sa.Index('ix_test', c),

    )

engine = sa.engine.create_engine("sqlite://", echo=True)

Base.metadata.create_all(engine)

session = sa.orm.Session(engine)

session.add_all([

    B()

])

session.commit()

print(session.query(A))

And it throws:

Traceback (most recent call last):

 File "test.py", line 29, in <module>

   class C(A):

 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla

rative/api.py", line 64, in __init__

   _as_declarative(cls, classname, cls.__dict__)

 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla

rative/base.py", line 88, in _as_declarative

   _MapperConfig.setup_mapping(cls, classname, dict_)

 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla

rative/base.py", line 103, in setup_mapping

   cfg_cls(cls_, classname, dict_)

 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla

rative/base.py", line 133, in __init__

   self._setup_inheritance()

 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla

rative/base.py", line 442, in _setup_inheritance

   "Can't place __table_args__ on an inherited class "

sqlalchemy.exc.ArgumentError: Can't place __table_args__ on an inherited class with no table.

Do somebody know any workarounds? 

[mike bayer]

there's a long term issue to provide some compatibility here at
https://bitbucket.org/zzzeek/sqlalchemy/issues/2700/support-__table_args__-on-single-inh.
For things like Index you can create those out of the table like this:

class Whatever(...):
..

Index('my_index', Whatever.some_column)


for UniqueConstraint, CheckConstraint, these also will "auto attach" in
modern versions if you create them in terms of the columns:

UniqueConstraint(Whatever.some_column)

If you use string names, you can append it:

uq = UniqueConstraint("some_column")
Whatever.__table__.append_constraint(uq)

On 05/07/2017 10:41 AM, Andrew Pashkin wrote:
> I've found that if __table_args__ is present on a child class in a
> single-table inheritance setup

> <http://docs.sqlalchemy.org/en/latest/orm/inheritance.html#single-table-inheritance>,

> SQLAlchemy throws sqlalchemy.exc.ArgumentError: Can't place
> __table_args__ on an inherited class with no table. At the same time it

> is possible to define a column with index=Trueon a child class, which

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[Andrew Pashkin]

Awesome, thank you for the response, this is what I was looking for.