Thanks
On Sunday, August 30, 2015 at 10:28:58 PM UTC-5, Michael Bayer wrote:
>
>
>
> On 8/30/15 1:58 PM, r...@rosenfeld.to <javascript:> wrote:
>
> Hi All,
>
> I'm trying to replace string definitions prgrammatically where possible.
> I have the following working:
>
> from sqlalchemy import Column
> from sqlalchemy import Integer
> from sqlalchemy import ForeignKey
> from sqlalchemy.orm import relationship
> from sqlalchemy.ext.declarative import declarative_base
>
> import pprint
>
>
> Base = declarative_base()
>
>
> class Parent(Base):
> __tablename__ = 'parent'
> id = Column(Integer, primary_key=True)
> children = relationship("Child", backref=__tablename__)
>
>
> class Child(Base):
> __tablename__ = 'child'
> id = Column(Integer, primary_key=True)
> parent_id = Column(Integer, ForeignKey('parent.id'))
>
>
> p = Parent()
> pprint.pprint('p table name: {}'.format(p.__tablename__))
> c = Child()
> pprint.pprint('c table name: {}'.format(c.__tablename__))
> c.parent = p
> pprint.pprint('Children: {}'.format(p.children))
> pprint.pprint('Parent: {}'.format(c.parent))
>
>
> But I'd like to follow the mixin example from the docs to inherit common
> columns and a function for __tablename__ via the Common class.
>
> from sqlalchemy import Column
> from sqlalchemy import Integer
> from sqlalchemy import ForeignKey
> from sqlalchemy.orm import relationship
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.ext.declarative import declared_attr
>
> import pprint
> from inflection import underscore
>
>
> class Common(object):
> @declared_attr
> def __tablename__(cls):
> return underscore(cls.__name__)
> id = Column(Integer, primary_key=True)
>
>
> Base = declarative_base(cls=Common)
>
>
> class Parent(Base):
> children = relationship("Child", backref=__tablename__)
>
>
> class Child(Base):
> parent_id = Column(Integer, ForeignKey('parent.id'))
>
>
> p = Parent()
> pprint.pprint('p table name: {}'.format(p.__tablename__))
> c = Child()
> pprint.pprint('c table name: {}'.format(c.__tablename__))
> c.parent = p
> pprint.pprint('Children: {}'.format(p.children))
> pprint.pprint('Parent: {}'.format(c.parent))
>
>
>
> I cannot figure out how to refer to the __tablename__ property in backref
> using this model.
>
> well it needs to be a callable function that is called when the "cls", in
> this case Parent, is available. It would be awkward but you'd need to do
> it like this:
>
> @declared_attr
> def children(cls):
> return relationship("Child", backref=cls.__tablename__)
>
> this is because the way it is now, "children = relationship" needs to know
> the name "Parent" which is explicitly not available yet, because you're in
> a class block named "Parent" - that name isn't known to the script until
> you exit that block. relationship() allows a lot of its members to be
> passed as functions but backref / back_populates aren't one of them because
> these have to do with the class mapping, rather than tables/columns which
> is why the deferred approach is provided.
>
> You can make your own function that generates that @declared_attr in one
> step to save on verbosity.
>
>
>
>
>
> If I refer to Base.__tablename__ or Common.__tablename__, it gets the
> wrong value. Can you help me fix this?
>
> Thanks,
> Rob
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