==============
Custom lookups
==============

.. versionadded:: 1.7

.. module:: django.db.models.lookups
   :synopsis: Custom lookups

.. currentmodule:: django.db.models

By default Django offers a wide variety of :ref:`built-in lookups
<field-lookups>` for filtering (for example, ``exact`` and ``icontains``). This
documentation explains how to write custom lookups and how to alter the working
of existing lookups.

A simple Lookup example
~~~~~~~~~~~~~~~~~~~~~~~

Let's start with a simple custom lookup. We will write a custom lookup ``ne``
which works opposite to ``exact``. ``Author.objects.filter(name__ne='Jack')``
will translate to the SQL::

  "author"."name" <> 'Jack'

This SQL is backend independent, so we don't need to worry about different
databases.

There are two steps to making this work. Firstly we need to implement the
lookup, then we need to tell Django about it. The implementation is quite
straightforward::

  from django.db.models import Lookup

  class NotEqual(Lookup):
      lookup_name = 'ne'

      def as_sql(self, qn, connection):
          lhs, lhs_params = self.process_lhs(qn, connection)
          rhs, rhs_params = self.process_rhs(qn, connection)
          params = lhs_params + rhs_params
          return '%s <> %s' % (lhs, rhs), params

To register the ``NotEqual`` lookup we will just need to call
``register_lookup`` on the field class we want the lookup to be available. In
this case, the lookup makes sense on all ``Field`` subclasses, so we register
it with ``Field`` directly::

  from django.db.models.fields import Field
  Field.register_lookup(NotEqual)

We can now use ``foo__ne`` for any field ``foo``. You will need to ensure that
this registration happens before you try to create any querysets using it. You
could place the implementation in a ``models.py`` file, or register the lookup
in the ``ready()`` method of an ``AppConfig``.

Taking a closer look at the implementation, the first required attribute is
``lookup_name``. This allows the ORM to understand how to interpret ``name__ne``
and use ``NotEqual`` to generate the SQL. By convention, these names are always
lowercase strings containing only letters, but the only hard requirement is
that it must not contain the string ``__``.

A ``Lookup`` works against two values, ``lhs`` and ``rhs``, standing for
left-hand side and right-hand side. The left-hand side is usually a field
reference, but it can be anything implementing the :ref:`query expression API
<query-expression>`. The right-hand is the value given by the user. In the
example ``Author.objects.filter(name__ne='Jack')``, the left-hand side is a
reference to the ``name`` field of the ``Author`` model, and ``'Jack'`` is the
right-hand side.

We call ``process_lhs`` and ``process_rhs`` to convert them into the values we
need for SQL. In the above example, ``process_lhs`` returns
``('"author"."name"', [])`` and ``process_rhs`` returns ``('"%s"', ['Jack'])``.
In this example there were no parameters for the left hand side, but this would
depend on the object we have, so we still need to include them in the
parameters we return.

Finally we combine the parts into a SQL expression with ``<>``, and supply all
the parameters for the query. We then return a tuple containing the generated
SQL string and the parameters.

A simple transformer example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The custom lookup above is great, but in some cases you may want to be able to
chain lookups together. For example, let's suppose we are building an
application where we want to make use of the ``abs()`` operator.
We have an ``Experiment`` model which records a start value, end value and the
change (start - end). We would like to find all experiments where the change
was equal to a certain amount (``Experiment.objects.filter(change__abs=27)``),
or where it did not exceede a certain amount
(``Experiment.objects.filter(change__abs__lt=27)``).

.. note::
    This example is somewhat contrived, but it demonstrates nicely the range of
    functionality which is possible in a database backend independent manner,
    and without duplicating functionality already in Django.

We will start by writing a ``AbsoluteValue`` transformer. This will use the SQL
function ``ABS()`` to transform the value before comparison::

  from django.db.models import Transform

  class AbsoluteValue(Transform):
      lookup_name = 'abs'

      def as_sql(self, qn, connection):
          lhs, params = qn.compile(self.lhs)
          return "ABS(%s)" % lhs, params

Next, lets register it for ``IntegerField``::

  from django.db.models import IntegerField
  IntegerField.register_lookup(AbsoluteValue)

We can now run the queris we had before.
``Experiment.objects.filter(change__abs=27)`` will generate the following SQL::

    SELECT ... WHERE ABS("experiments"."change") = 27

By using ``Transform`` instead of ``Lookup`` it means we are able to chain
further lookups afterwards. So
``Experiment.objects.filter(change__abs__lt=27)`` will generate the following
SQL::

    SELECT ... WHERE ABS("experiments"."change") < 27

Subclasses of ``Transform`` usually only operate on the left-hand side of the
expression. Further lookups will work on the transformed value. Note that in
this case where there is no other lookup specified, Django interprets
``change__abs=27`` as ``change__abs__exact=27``.

When looking for which lookups are allowable after the ``Transform`` has been
applied, Django uses the ``output_type`` attribute. We didn't need to specify
this here as it didn't change, but supposing we were applying ``AbsoluteValue``
to some field which represents a more complex type (for example a point
relative to an origin, or a complex number) then we may have wanted to specify
``output_type = FloatField``, which will ensure that further lookups like
``abs__lte`` behave as they would for a ``FloatField``.

Writing an efficient abs__lt lookup
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When using the above written ``abs`` lookup, the SQL produced will not use
indexes efficiently in some cases. In particular, when we use
``change__abs__lt=27``, this is equivalent to ``change__gt=-27`` AND
``change__lt=27``. (For the ``lte`` case we could use the SQL ``BETWEEN``).

So we would like ``Experiment.objects.filter(change__abs__lt=27)`` to generate
the following SQL::

    SELECT .. WHERE "experiments"."change" < 27 AND "experiments"."change" > -27

The implementation is::

  from django.db.models import Lookup

  class AbsoluteValueLessThan(Lookup):
      lookup_name = 'lt'

      def as_sql(self, qn, connection):
          lhs, lhs_params = qn.compile(self.lhs.lhs)
          rhs, rhs_params = self.process_rhs(qn, connection)
          params = lhs_params + rhs_params + lhs_params + rhs_params
          return '%s > %s AND %s < -%s % (lhs, rhs, lhs, rhs), params

  AbsoluteValue.register_lookup(AbsoluteValueLessThan)

There are a couple of notable things going on. First, ``AbsoluteValueLessThan``
isn't calling ``process_lhs()``. Instead it skips the transformation of the
``lhs`` done by ``AbsoluteValue`` and uses the original ``lhs``. That is, we
want to get ``27`` not ``ABS(27)``. Referring directly to ``self.lhs.lhs`` is
safe as ``AbsoluteValueLessThan`` can be accessed only from the
``AbsoluteValue`` lookup, that is the ``lhs`` is always an instance of
``AbsoluteValue``.

Notice also that  as both sides are used multiple times in the query the params
need to contain ``lhs_params`` and ``rhs_params`` multiple times.

The final query does the inversion (``27`` to ``-27``) directly in the
database. The reason for doing this is that if the self.rhs is something else
than a plain integer value (for example an ``F()`` reference) we can't do the
transformations in Python.

.. note::
    In fact, most lookups with ``__abs`` could be implemented as range queries
    like this, and on most database backend it is likely to be more sensible to
    do so as you can make use of the indexes. However with PostgreSQL you may
    want to add an index on ``abs(change)`` which would allow these queries to
    be very efficient.

Writing alternative implemenatations for existing lookups
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes different database vendors require different SQL for the same
operation. For this example we will rewrite a custom implementation for
MySQL for the NotEqual operator. Instead of ``<>`` we will be using ``!=``
operator. (Note that in reality almost all databases support both, including
all the official databases supported by Django).

We can change the behaviour on a specific backend by creating a subclass of
``NotEqual`` with a ``as_mysql`` method::

  class MySQLNotEqual(NotEqual):
      def as_mysql(self, qn, connection):
          lhs, lhs_params = self.process_lhs(qn, connection)
          rhs, rhs_params = self.process_rhs(qn, connection)
          params = lhs_params + rhs_params
          return '%s != %s' % (lhs, rhs), params
  Field.register_lookup(MySQLNotExact)

We can then register it with ``Field``. It takes the place of the original
``NotEqual`` class as it has 

When compiling a query, Django first looks for ``as_%s % connection.vendor``
methods, and then falls back to ``as_sql``. The vendor names for the in-built
backends are ``sqlite``, ``postgresql``, ``oracle`` and ``mysql``.

.. _query-expression:

The Query Expression API
~~~~~~~~~~~~~~~~~~~~~~~~

A lookup can assume that the lhs responds to the query expression API.
Currently direct field references, aggregates and ``Transform`` instances respond
to this API.

.. method:: as_sql(qn, connection)

    Responsible for producing the query string and parameters for the
    expression. The ``qn`` has a ``compile()`` method that can be used to
    compile other expressions. The ``connection`` is the connection used to
    execute the query.

    Calling expression.as_sql() directly is usually incorrect - instead
    qn.compile(expression) should be used. The qn.compile() method will take
    care of calling vendor-specific methods of the expression.

.. method:: get_lookup(lookup_name)

    The ``get_lookup()`` method is used to fetch lookups. By default the
    lookup is fetched from the expression's output type in the same way
    described in registering and fetching lookup documentation below.
    It is possible to override this method to alter that behaviour.

.. method:: as_vendorname(qn, connection)

    Works like ``as_sql()`` method. When an expression is compiled by
    ``qn.compile()``, Django will first try to call ``as_vendorname()``, where
    vendorname is the vendor name of the backend used for executing the query.
    The vendorname is one of ``postgresql``, ``oracle``, ``sqlite`` or
    ``mysql`` for Django's built-in backends.

.. attribute:: output_type

    The ``output_type`` attribute is used by the ``get_lookup()`` method to check for
    lookups. The output_type should be a field.

Note that this documentation lists only the public methods of the API.

Lookup reference
~~~~~~~~~~~~~~~~

.. class:: Lookup

    In addition to the attributes and methods below, lookups also support
    ``as_sql`` and ``as_vendorname`` from the query expression API.

.. attribute:: lhs

    The ``lhs`` (left-hand side) of a lookup tells us what we are comparing the
    rhs to. It is an object which implements the query expression API. This is
    likely to be a field, an aggregate or a subclass of ``Transform``.

.. attribute:: rhs

    The ``rhs`` (right-hand side) of a lookup is the value we are comparing the
    left hand side to. It may be a plain value, or something which compiles
    into SQL, for example an ``F()`` object or a ``Queryset``.

.. attribute:: lookup_name

    This class level attribute is used when registering lookups. It determines
    the name used in queries to trigger this lookup. For example, ``contains``
    or ``exact``. This should not contain the string ``__``.

.. method:: process_lhs(qn, connection)

    This returns a tuple of ``(lhs_string, lhs_params)``. In some cases you may
    wish to compile ``lhs`` directly in your ``as_sql`` methods using
    ``qn.compile(self.lhs)``.

.. method:: process_rhs(qn, connection)

    Behaves the same as ``process_lhs`` but acts on the right-hand side.

Transform reference
~~~~~~~~~~~~~~~~~~~

.. class:: Transform

    In addition to implementing the query expression API Transforms have the
    following methods and attributes.

.. attribute:: lhs

    The ``lhs`` (left-hand-side) of a transform contains the value to be
    transformed. The ``lhs`` implements the query expression API.

.. attribute:: lookup_name

    This class level attribute is used when registering lookups. It determines
    the name used in queries to trigger this lookup. For example, ``year``
    or ``dayofweek``. This should not contain the string ``__``.

.. _lookup-registration-api:

Registering and fetching lookups
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The lookup registration API is explained below.

.. classmethod:: register_lookup(lookup)

    Registers the Lookup or Transform for the class. For example
    ``DateField.register_lookup(YearExact)`` will register ``YearExact`` for
    all ``DateFields`` in the project, but also for fields that are instances
    of a subclass of ``DateField`` (for example ``DateTimeField``).

.. method:: get_lookup(lookup_name)

    Django uses ``get_lookup(lookup_name)`` to fetch lookups or transforms.
    The implementation of ``get_lookup()`` fetches lookups or transforms
    registered for the current class based on their lookup_name attribute.

The lookup registration API is available for ``Transform`` and ``Field`` classes.