Fixed #2064 -- Added 'Falling back to raw SQL' section to docs/db-api.txt

For every ``ImageField``, the object will have ``get_FOO_height()`` and

``get_FOO_width()`` methods, where ``FOO`` is the name of the field. This

returns the height (or width) of the image, as an integer, in pixels.

Falling back to raw SQL

=======================

If you find yourself needing to write an SQL query that is too complex for

Django's database-mapper to handle, you can fall back into raw-SQL statement

mode.

The preferred way to do this is by giving your model custom methods or custom

manager methods that execute queries. Although there's nothing in Django that

*requires* database queries to live in the model layer, this approach keeps all

your data-access logic in one place, which is smart from an code-organization

standpoint. For instructions, see `Executing custom SQL`_.

Finally, it's important to note that the Django database layer is merely an

interface to your database. You can access your database via other tools,

programming languages or database frameworks; there's nothing Django-specific

about your database.

.. _Executing custom SQL: http://www.djangoproject.com/documentation/model_api/#executing-custom-sql