app/django/db/models/fields/subclassing.py
author Pawel Solyga <Pawel.Solyga@gmail.com>
Tue, 27 Jan 2009 21:56:32 +0000
changeset 1015 b9d51be5104a
parent 323 ff1a9aa48cfd
permissions -rw-r--r--
Add profiling support to Melange. By assigning profile_main_as_logs or profile_main_as_html to main variable you can turn on profiling. profile_main_as_logs will log profile data to App Engine console logs, profile_main_as_html will show profile data as html at the bottom of the page. If you want to profile app on deployed app just set the profiling function and deploy it. Patch by: Pawel Solyga Reviewed by: to-be-reviewed

"""
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