[Sqlalchemy-tickets] Issue #4014: SQLA don't set FK in relationship if inheritance present (zzzeek/sqlalchemy)

[Вася Г. <iss...@bi...>]

New issue 4014: SQLA don't set FK in relationship if inheritance present
https://bitbucket.org/zzzeek/sqlalchemy/issues/4014/sqla-dont-set-fk-in-relationship-if

Вася Гайкин:

I want build pseudo file system on DB tables and make several classes. But assign to **item.parent** attribute has no effect. It work if write **item.parent_id = parent.id**.

*(in attanchment simple example project)*

```
#!python

class Item( Base ):
    name = Column( String, nullable=False )

    parent_id = Column( Integer,
                        ForeignKey( 'Item.id',
                                    onupdate='cascade',
                                    ondelete='cascade',
                                    deferrable=True ),
                        nullable=False )

    parent = relationship( 'Folder',
                           foreign_keys=parent_id,
                           uselist=False,
                           post_update=True )

    _type = Column( String, nullable=False )
    __mapper_args__ = { 'polymorphic_on': _type }


class File( Item ):
    id = Column( Integer,
                 ForeignKey( 'Item.id',
                             onupdate='cascade',
                             ondelete='cascade' ),
                 primary_key=True )

    data = Column( LargeBinary, nullable=False )
    mime = Column( String, nullable=False )

    preview_id = Column( Integer,
                         ForeignKey( 'File.id',
                                     onupdate='cascade',
                                     ondelete='cascade' ) )

    preview = relationship( 'File', foreign_keys=preview_id, uselist=False )

    __mapper_args__ = { 'polymorphic_identity': 'file' }
```

**part of  utils.py**

```
#!python

def load_file( path, parent ):
    data = open( path, 'rb' ).read()
    name = os.path.basename( path )

    file = File()
    file.name = name
    file.data = data
    file.mime = 'image/jpeg'

    file.parent_id = parent.id  # <--- OK
    # file.parent  = parent # <--- FAIL

    session.add( file )
    session.commit()

    return file
```

**part of run.py**

```
#!python

    main_file = load_file( '/tmp/example_2.jpg', usr )
    preview = load_file( '/tmp/example_3.jpg', bin_ )

    main_file.preview = preview  # <--- OK

    link = Link()
    link.target = root
    link.name = main_file.name
    # link.parent_id = root.id  # <--- OK
    link.parent = root  # <--- FAIL
    session.add( link )

    session.commit()
```