app/django/db/models/fields/subclassing.py
author Pawel Solyga <Pawel.Solyga@gmail.com>
Mon, 01 Jun 2009 22:23:46 +0200
changeset 2376 feec28b50f1b
parent 323 ff1a9aa48cfd
permissions -rw-r--r--
Extend taggable-mixin to support different Tag models. Usage is pretty simple. Tag model is default in Taggable constructor but you can call it with different model like GHOPTaskType that inherits from Tag model. Both Taggable and Tag models have been updated and they don't use hardcoded Tag model anymore and instead use cls of class methods or self.__class__. In case of Taggable it's self.__tag_model.

"""
Convenience routines for creating non-trivial Field subclasses.

Add SubfieldBase as the __metaclass__ for your Field subclass, implement
to_python() and the other necessary methods and everything will work seamlessly.
"""

class SubfieldBase(type):
    """
    A metaclass for custom Field subclasses. This ensures the model's attribute
    has the descriptor protocol attached to it.
    """
    def __new__(cls, base, name, attrs):
        new_class = super(SubfieldBase, cls).__new__(cls, base, name, attrs)
        new_class.contribute_to_class = make_contrib(
                attrs.get('contribute_to_class'))
        return new_class

class Creator(object):
    """
    A placeholder class that provides a way to set the attribute on the model.
    """
    def __init__(self, field):
        self.field = field

    def __get__(self, obj, type=None):
        if obj is None:
            raise AttributeError('Can only be accessed via an instance.')
        return obj.__dict__[self.field.name]        

    def __set__(self, obj, value):
        obj.__dict__[self.field.name] = self.field.to_python(value)

def make_contrib(func=None):
    """
    Returns a suitable contribute_to_class() method for the Field subclass.

    If 'func' is passed in, it is the existing contribute_to_class() method on
    the subclass and it is called before anything else. It is assumed in this
    case that the existing contribute_to_class() calls all the necessary
    superclass methods.
    """
    def contribute_to_class(self, cls, name):
        if func:
            func(self, cls, name)
        else:
            super(self.__class__, self).contribute_to_class(cls, name)
        setattr(cls, self.name, Creator(self))

    return contribute_to_class