File storage refactoring, adding far more flexibility to Django's file handling. The new files.txt document has details of the new features.

This is a backwards-incompatible change; consult BackwardsIncompatibleChanges for details.

Fixes #3567, #3621, #4345, #5361, #5655, #7415.

Many thanks to Marty Alchin who did the vast majority of this work.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8244 bcc190cf-cafb-0310-a4f2-bffc1f526a37

1 files changed, 39 insertions, 0 deletions

diff --git a/docs/custom_model_fields.txt b/docs/custom_model_fields.txt
index 6b8f3c3ac6..a45e876fc6 100644
--- a/docs/custom_model_fields.txt
+++ b/docs/custom_model_fields.txt
@@ -596,3 +596,42 @@ smoothly:
        instance, not a ``HandField``). So if your ``__unicode__()`` method
        automatically converts to the string form of your Python object, you can
        save yourself a lot of work.
+
+Writing a ``FileField`` subclass
+=================================
+
+In addition to the above methods, fields that deal with files have a few other
+special requirements which must be taken into account. The majority of the
+mechanics provided by ``FileField``, such as controlling database storage and
+retrieval, can remain unchanged, leaving subclasses to deal with the challenge
+of supporting a particular type of file.
+
+Django provides a ``File`` class, which is used as a proxy to the file's
+contents and operations. This can be subclassed to customzie hwo the file is
+accessed, and what methods are available. It lives at
+``django.db.models.fields.files``, and its default behavior is explained in the
+`file documentation`_.
+
+Once a subclass of ``File`` is created, the new ``FileField`` subclass must be
+told to use it. To do so, simply assign the new ``File`` subclass to the special
+``attr_class`` attribute of the ``FileField`` subclass.
+
+.. _file documentation: ../files/
+
+A few suggestions
+------------------
+
+In addition to the above details, there are a few guidelines which can greatly
+improve the efficiency and readability of the field's code.
+
+    1. The source for Django's own ``ImageField`` (in
+       ``django/db/models/fields/files.py``) is a great example of how to
+       subclass ``FileField`` to support a particular type of file, as it
+       incorporates all of the techniques described above.
+
+    2. Cache file attributes wherever possible. Since files may be stored in
+       remote storage systems, retrieving them may cost extra time, or even
+       money, that isn't always necessary. Once a file is retrieved to obtain
+       some data about its content, cache as much of that data as possible to
+       reduce the number of times the file must be retrieved on subsequent
+       calls for that information.