Fixed #25687 -- Documented how to add database function support to third-party backends.

Thanks Kristof Claes for the initial patch.
This commit is contained in:
Tim Graham 2016-02-12 14:43:15 -05:00
parent f0425c7260
commit 88034c9938
1 changed files with 31 additions and 0 deletions

View File

@ -640,3 +640,34 @@ Let's see how it works::
Apple: AAPL Apple: AAPL
Yahoo: Internet Company Yahoo: Internet Company
Django Software Foundation: No Tagline Django Software Foundation: No Tagline
Adding support in third-party database backends
-----------------------------------------------
If you're using a database backend that uses a different SQL syntax for a
certain function, you can add support for it by monkey patching a new method
onto the function's class.
Let's say we're writing a backend for Microsoft's SQL Server which uses the SQL
``LEN`` instead of ``LENGTH`` for the :class:`~functions.Length` function.
We'll monkey patch a new method called ``as_sqlserver()`` onto the ``Length``
class::
from django.db.models.functions import Length
def sqlserver_length(self, compiler, connection):
return self.as_sql(compiler, connection, function='LEN')
Length.as_sqlserver = sqlserver_length
You can also customize the SQL using the ``template`` parameter of ``as_sql()``.
We use ``as_sqlserver()`` because ``django.db.connection.vendor`` returns
``sqlserver`` for the backend.
Third-party backends can register their functions in the top level
``__init__.py`` file of the backend package or in a top level ``expressions.py``
file (or package) that is imported from the top level ``__init__.py``.
For user projects wishing to patch the backend that they're using, this code
should live in an :meth:`AppConfig.ready()<django.apps.AppConfig.ready>` method.