This patch gives FileField (and ImageField) objects the ability to be used as 'core' fields in related models. Also added is the ability to clear the field, deleting the uploaded file from the filesytem.
FileField and ImageField objects do not work properly as core fields. This seems to be acknowledged in manipulators.py, where they are treated as a special case, but as a result, if a FileField or ImageField is the only field marked core=True in a related model, then that record will always be saved, even if every field is empty. This patch corrects this issue by providing a get_manipulator_new_data method for FileField which returns the new value that the field will take, rather than the pre-existing value.
Since core=True fields aren't very useful without the ability to clear them, this patch also adds the optional ability to delete files after they've been uploaded. This has been long-standing issue in Django -- see, for example, Ticket #22. The patch adds a new optional constructor argument, can_delete, which, when set, will place a checkbox labeled Delete on the HTML form. When the user checks this checkbox and saves the form, the FileField will be cleared and the file associated with the field will be removed from the filesystem. (Unlike the patch in ticket #22 that provides an HTML link which redirects the user away from the change form, the Delete checkbox in this patch does not get processed until the user saves the object. This also means that the user can choose to clear a FileField, make other changes to an object, and save all of the changes atomically.)
In creating this patch, we have strived to change as little code as possible. We hope that it will be accepted into Django as it immediately addresses a significant shortcoming while still maintaining compatibility with existing models and apps.
This patch is largely based on our work at http://www.verdjn.com/wiki/FileField, but goes further by actually allowing FileField objects to be used as core fields.
