[1.9.x] Fixed #26702 -- Documented how to change the base class of a custom field.

    new_instance = MyField(*args, **kwargs)

    self.assertEqual(my_field_instance.some_attribute, new_instance.some_attribute)

Changing a custom field's base class

------------------------------------

You can't change the base class of a custom field because Django won't detect

the change and make a migration for it. For example, if you start with::

    class CustomCharField(models.CharField):

        ...

and then decide that you want to use ``TextField`` instead, you can't change

the subclass like this::

    class CustomCharField(models.TextField):

        ...

Instead, you must create a new custom field class and update your models to

reference it::

    class CustomCharField(models.CharField):

        ...

    class CustomTextField(models.TextField):

        ...

As discussed in :ref:`removing fields <migrations-removing-model-fields>`, you

must retain the original ``CustomCharField`` class as long as you have

migrations that reference it.

Documenting your custom field

-----------------------------