app/django/db/models/fields/subclassing.py
author Todd Larsen <tlarsen@google.com>
Wed, 15 Oct 2008 17:10:27 +0000
changeset 339 b9be44e09530
parent 323 ff1a9aa48cfd
permissions -rw-r--r--
Define the Models for implementing Quizzes (collections of Questions) and their Responses (collections of Answers to those Questions). These Models would form the basis of storage for such items as: Terms of Service (Quiz) Question ("I agree...") Response -> Answer (answer to "I agree..." confirmation) solution ("Yes" Answer to the "I agree..." Question) Mentor and Student surveys (Quiz) Questions (including "Pay this student?") Response -> Answers solution ("Yes" Answer to the "Pay this student?" Question) Organization applications Student Proposal review, comment, and scoring system GHOP task tracking (a specific task list item would be a Quiz) Patch by: Todd Larsen Review by: Pawel Solyga, Sverre Rabbelier, Chen Lunpeng Review URL: http://codereviews.googleopensourceprograms.com/1403

"""
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