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Fixed #25687 -- Documented how to add database function support to third-party backends.
Thanks Kristof Claes for the initial patch.
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@ -640,3 +640,34 @@ Let's see how it works::
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Apple: AAPL
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Yahoo: Internet Company
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Django Software Foundation: No Tagline
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Adding support in third-party database backends
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-----------------------------------------------
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If you're using a database backend that uses a different SQL syntax for a
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certain function, you can add support for it by monkey patching a new method
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onto the function's class.
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Let's say we're writing a backend for Microsoft's SQL Server which uses the SQL
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``LEN`` instead of ``LENGTH`` for the :class:`~functions.Length` function.
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We'll monkey patch a new method called ``as_sqlserver()`` onto the ``Length``
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class::
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from django.db.models.functions import Length
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def sqlserver_length(self, compiler, connection):
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return self.as_sql(compiler, connection, function='LEN')
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Length.as_sqlserver = sqlserver_length
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You can also customize the SQL using the ``template`` parameter of ``as_sql()``.
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We use ``as_sqlserver()`` because ``django.db.connection.vendor`` returns
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``sqlserver`` for the backend.
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Third-party backends can register their functions in the top level
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``__init__.py`` file of the backend package or in a top level ``expressions.py``
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file (or package) that is imported from the top level ``__init__.py``.
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For user projects wishing to patch the backend that they're using, this code
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should live in an :meth:`AppConfig.ready()<django.apps.AppConfig.ready>` method.
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