Generic foreign key field for peewee based on postgresql's jsonb type, with Inter-Model-Identifier(IMID) support.

fields.py

import six
from peewee import Model, FieldAccessor
from playhouse.postgres_ext import BinaryJSONField


class GForeignKeyAccessor(FieldAccessor):
    def get_rel_instance(self, instance):
        value = instance.__data__.get(self.name)
        model_name = value['model']
        rel_model = self.field.allowed_types[model_name]
        rel_field_name = value.get('rel_field') or self.field.get_ref_field(model_name).name
        rel_field = getattr(rel_model, rel_field_name)

        if value is not None or self.name in instance.__rel__:
            if self.name not in instance.__rel__:
                obj = rel_model.get(rel_field == value.get('pk', value.get('id')))
                instance.__rel__[self.name] = obj
            return instance.__rel__[self.name]
        elif not self.field.null:
            raise rel_model.DoesNotExist
        return value

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return self.get_rel_instance(instance)
        return self.field

    def __set__(self, instance, obj):
        if isinstance(obj, tuple(self.field.allowed_types.values())):
            # e.g. fm.subject = compound
            instance.__data__[self.name] = self.field.db_value(obj).adapted
            instance.__rel__[self.name] = obj
        elif isinstance(obj, six.string_types):
            # e.g. fm.subject = 'Compound/7'
            data = self.field.db_value(obj).adapted
            prev_value = instance.__data__.get(self.name)
            instance.__data__[self.name] = data
            if data != prev_value and self.name in instance.__rel__:
                del instance.__rel__[self.name]
        else:
            # while loading from db
            # reference: http://initd.org/psycopg/docs/extras.html#json-adaptation
            prev_value = instance.__data__.get(self.name)
            instance.__data__[self.name] = obj
            if obj != prev_value and self.name in instance.__rel__:
                del instance.__rel__[self.name]

        instance._dirty.add(self.name)


class GForeignKeyField(BinaryJSONField):
    accessor_class = GForeignKeyAccessor

    def __init__(self, allowed_types=None, *args, **kwargs):
        super(GForeignKeyField, self).__init__(*args, **kwargs)
        self.allowed_types = {m.__name__: m for m in allowed_types or []}

    def get_ref_field(self, model):
        if isinstance(model, six.string_types):
            if model not in self.allowed_types:
                raise TypeError('{} not in in {}'.format(model, self.allowed_types))
            model = self.allowed_types[model]
        return getattr(model, 'id')

    def db_value(self, value):
        if isinstance(value, Model):
            model = value._meta.model
            ref_field = self.get_ref_field(model)
            value = {
                'model': model.__name__,
                'pk': getattr(value, ref_field.name),
            }
        elif isinstance(value, six.string_types):
            model_name, pk = value.split('/')
            ref_field = self.get_ref_field(model_name)
            value = {
                'model': model_name,
                'pk': ref_field.db_value(pk),
            }

        return super(GForeignKeyField, self).db_value(value)

    def __eq__(self, rhs):
        return self.contains(self.db_value(rhs).adapted)

    def __ne__(self, rhs):
        return ~(self.__eq__(rhs))

index.md

HOW TO USE

Sample Models

class Post(Model):
    pass


class News(Model):
    pass


class Comment(Model):
    of = GForeignKeyField(allowed_types={Post, News})

Create Comments

p1 = Post(); p1.save()
n1 = News(); n1.save()

Comment(of=p1).save()
Comment(of='News/1').save()  # By IMID notation

Retrieve/Query Comments by target

c1 = Comment.get(Comment.of == p1)
c2 = Comment.get(Comment.of == 'News/1)  # By IMID notation
print(Comment.select().where(Comment.of == n1).sql())
print(Comment.select().where(Comment.of == 'News/1').sql())

Get/Set Comment's target object

print(c1.of)
c1.of = n1
c1.of = 'News/1'  # or By IMID notation
c1.save()