From 88034c9938d92193d2104ecfe77999c69301dcc1 Mon Sep 17 00:00:00 2001 From: Tim Graham Date: Fri, 12 Feb 2016 14:43:15 -0500 Subject: [PATCH] Fixed #25687 -- Documented how to add database function support to third-party backends. Thanks Kristof Claes for the initial patch. --- docs/ref/models/expressions.txt | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) diff --git a/docs/ref/models/expressions.txt b/docs/ref/models/expressions.txt index 98ba323171..f48dcbbe36 100644 --- a/docs/ref/models/expressions.txt +++ b/docs/ref/models/expressions.txt @@ -640,3 +640,34 @@ Let's see how it works:: Apple: AAPL Yahoo: Internet Company 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()` method.