I am currently trying to create the following database schema with SQLAlchemy (using ext.declarative):
I have a base class MyBaseClass
which provides some common functionality for all of my publicly accessible classes, a mixin class MetadataMixin
that provides functionality to query metadata from imdb and store it.
Every class that subclasses MetadataMixin
has a field persons
which provides a M:N relationship to instances of the Person
class, and a field persons_roles
which provides a 1:N relationship to an object (one for each subclass) which stores the role
a concrete Person plays in the instance of the subclass.
This is an abbreviated version of what my code looks like at the moment:
from sqlalchemy import Column, Integer, Enum, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class MyBaseClass(object):
"""Base class for all publicly accessible classes"""
id = Column(Integer, primary_key=True)
class Person(MyBaseClass):
"""A Person"""
name = Column(Unicode)
movies = association_proxy('movie_roles', 'movie',
creator=lambda m: _PersonMovieRole(movie=m))
shows = association_proxy('show_roles', 'show',
creator=lambda s: _PersonShowRole(show=s=))
class _PersonMovieRole(Base):
"""Role for a Person in a Movie"""
__tablename__ = 'persons_movies'
id = Column(Integer, primary_key=True)
role = Column(Enum('none', 'actor', 'writer', 'director', 'producer'),
default='none')
person_id = Column(Integer, ForeignKey('persons.id'))
person = relationship('Person', backref='movie_roles')
movie_id = Column(Integer, ForeignKey('movies.id'))
movie = relationship('Movie', backref='persons_roles')
class _PersonShowRole(Base):
"""Role for a Person in a Show"""
__tablename__ = 'persons_shows'
id = Column(Integer, primary_key=True)
role = Column(Enum('none', 'actor', 'writer', 'director', 'producer'),
default='none')
person_id = Column(Integer, ForeignKey('persons.id'))
person = relationship('Person', backref='show_roles')
show_id = Column(Integer, ForeignKey('shows.id'))
show = relationship('Episode', backref='persons_roles')
class MetadataMixin(object):
"""Mixin class that provides metadata-fields and methods"""
# ...
persons = association_proxy('persons_roles', 'person',
creator= #...???...#)
class Movie(Base, MyBaseClass, MetadataMixin):
#....
pass
What I'm trying to do is to create a generic creator
function for association_proxy
that creates either a PersonMovieRole or a PersonShowRole object, depending on the class of the concrete instance that a Person
is added to. What I'm stuck on at the moment is that I don't know how to pass the calling class to the creator function.
Is this possible, or is there maybe even an easier way for what I'm trying to accomplish?
By the time your persons
field is defined, you cannot really know what class it will end up in. Python takes up ready dictionaries of class members and creates classes out of them (via type.__new__
), but when it happens, those members are already fully defined.
So you need to provide the required information directly to the mixin, and tolerate the small duplication it will create in your code. I'd opt for interface similar to this one:
class Movie(Base, MyBaseClass, MetadataMixin('Movie')):
pass
(You cannot have MetadataMixin(Movie)
either, for the exact same reasons: Movie
requires its base classes to be completely defined by the time the class is created).
To implement such "parametrized class", simply use a function:
def MetadataMixin(cls_name):
"""Mixin class that provides metadata-fields and methods"""
person_role_cls_name = 'Person%sRole' % cls_name
person_role_cls = Base._decl_class_registry[person_role_cls_name]
class Mixin(object):
# ...
persons = association_proxy('persons_roles', 'person',
creator=person_role_cls)
return Mixin
This works because what we're looking up in Base._decl_class_registry
- the registry of all classes descending from your declarative base - is not the final class (e.g. Movie
), but the association object (e.g. PersonMovieRole
).
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