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====================
Lookup API reference
====================
.. module:: django.db.models.lookups
:synopsis: Lookups API
.. currentmodule:: django.db.models
This document has the API references of lookups, the Django API for building
the ``WHERE`` clause of a database query. To learn how to *use* lookups, see
:doc:`/topics/db/queries`; to learn how to *create* new lookups, see
:doc:`/howto/custom-lookups`.
The lookup API has two components: a :class:`~lookups.RegisterLookupMixin` class
that registers lookups, and the :ref:`Query Expression API <query-expression>`, a
set of methods that a class has to implement to be registrable as a lookup.
Django has two base classes that follow the query expression API and from where
all Django builtin lookups are derived:
* :class:`Lookup`: to lookup a field (e.g. the ``exact`` of ``field_name__exact``)
* :class:`Transform`: to transform a field
A lookup expression consists of three parts:
* Fields part (e.g. ``Book.objects.filter(author__best_friends__first_name...``);
* Transforms part (may be omitted) (e.g. ``__lower__first3chars__reversed``);
* A lookup (e.g. ``__icontains``) that, if omitted, defaults to ``__exact``.
.. _lookup-registration-api:
Registration API
~~~~~~~~~~~~~~~~
Django uses :class:`~lookups.RegisterLookupMixin` to give a class the interface to
register lookups on itself. The two prominent examples are
:class:`~django.db.models.Field`, the base class of all model fields, and
``Aggregate``, the base class of all Django aggregates.
.. class:: lookups.RegisterLookupMixin
A mixin that implements the lookup API on a class.
.. classmethod:: register_lookup(lookup, lookup_name=None)
Registers a new lookup in the class. For example
``DateField.register_lookup(YearExact)`` will register ``YearExact``
lookup on ``DateField``. It overrides a lookup that already exists with
the same name. ``lookup_name`` will be used for this lookup if
provided, otherwise ``lookup.lookup_name`` will be used.
.. versionchanged:: 1.9
The ``lookup_name`` parameter was added.
.. method:: get_lookup(lookup_name)
Returns the :class:`Lookup` named ``lookup_name`` registered in the class.
The default implementation looks recursively on all parent classes
and checks if any has a registered lookup named ``lookup_name``, returning
the first match.
.. method:: get_transform(transform_name)
Returns a :class:`Transform` named ``transform_name``. The default
implementation looks recursively on all parent classes to check if any
has the registered transform named ``transform_name``, returning the first
match.
For a class to be a lookup, it must follow the :ref:`Query Expression API
<query-expression>`. :class:`~Lookup` and :class:`~Transform` naturally
follow this API.
.. _query-expression:
The Query Expression API
~~~~~~~~~~~~~~~~~~~~~~~~
The query expression API is a common set of methods that classes define to be
usable in query expressions to translate themselves into SQL expressions. Direct
field references, aggregates, and ``Transform`` are examples that follow this
API. A class is said to follow the query expression API when it implements the
following methods:
.. method:: as_sql(self, compiler, connection)
Responsible for producing the query string and parameters for the expression.
The ``compiler`` is an ``SQLCompiler`` object, which 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()`` is usually incorrect - instead
``compiler.compile(expression)`` should be used. The ``compiler.compile()``
method will take care of calling vendor-specific methods of the expression.
.. method:: as_vendorname(self, compiler, connection)
Works like ``as_sql()`` method. When an expression is compiled by
``compiler.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.
.. method:: get_lookup(lookup_name)
Must return the lookup named ``lookup_name``. For instance, by returning
``self.output_field.get_lookup(lookup_name)``.
.. method:: get_transform(transform_name)
Must return the lookup named ``transform_name``. For instance, by returning
``self.output_field.get_transform(transform_name)``.
.. attribute:: output_field
Defines the type of class returned by the ``get_lookup()`` method. It must
be a :class:`~django.db.models.Field` instance.
Transform reference
~~~~~~~~~~~~~~~~~~~
.. class:: Transform
A ``Transform`` is a generic class to implement field transformations. A
prominent example is ``__year`` that transforms a ``DateField`` into a
``IntegerField``.
The notation to use a ``Transform`` in an lookup expression is
``<expression>__<transformation>`` (e.g. ``date__year``).
This class follows the :ref:`Query Expression API <query-expression>`, which
implies that you can use ``<expression>__<transform1>__<transform2>``. It's
a specialized :ref:`Func() expression <func-expressions>` that only accepts
one argument. It can also be used on the right hand side of a filter or
directly as an annotation.
.. versionchanged:: 1.9
``Transform`` is now a subclass of ``Func``.
.. attribute:: bilateral
A boolean indicating whether this transformation should apply to both
``lhs`` and ``rhs``. Bilateral transformations will be applied to ``rhs`` in
the same order as they appear in the lookup expression. By default it is set
to ``False``. For example usage, see :doc:`/howto/custom-lookups`.
.. attribute:: lhs
The left-hand side - what is being transformed. It must follow the
:ref:`Query Expression API <query-expression>`.
.. attribute:: lookup_name
The name of the lookup, used for identifying it on parsing query
expressions. It cannot contain the string ``"__"``.
.. attribute:: output_field
Defines the class this transformation outputs. It must be a
:class:`~django.db.models.Field` instance. By default is the same as
its ``lhs.output_field``.
Lookup reference
~~~~~~~~~~~~~~~~
.. class:: Lookup
A ``Lookup`` is a generic class to implement lookups. A lookup is a query
expression with a left-hand side, :attr:`lhs`; a right-hand side,
:attr:`rhs`; and a ``lookup_name`` that is used to produce a boolean
comparison between ``lhs`` and ``rhs`` such as ``lhs in rhs`` or
``lhs > rhs``.
The notation to use a lookup in an expression is
``<lhs>__<lookup_name>=<rhs>``.
This class doesn't follow the :ref:`Query Expression API <query-expression>`
since it has ``=<rhs>`` on its construction: lookups are always the end of
a lookup expression.
.. attribute:: lhs
The left-hand side - what is being looked up. The object must follow
the :ref:`Query Expression API <query-expression>`.
.. attribute:: rhs
The right-hand side - what ``lhs`` is being compared against. It can be
a plain value, or something that compiles into SQL, typically an
``F()`` object or a ``QuerySet``.
.. attribute:: lookup_name
The name of this lookup, used to identify it on parsing query
expressions. It cannot contain the string ``"__"``.
.. method:: process_lhs(compiler, connection, lhs=None)
Returns a tuple ``(lhs_string, lhs_params)``, as returned by
``compiler.compile(lhs)``. This method can be overridden to tune how
the ``lhs`` is processed.
``compiler`` is an ``SQLCompiler`` object, to be used like
``compiler.compile(lhs)`` for compiling ``lhs``. The ``connection``
can be used for compiling vendor specific SQL. If ``lhs`` is not
``None``, use it as the processed ``lhs`` instead of ``self.lhs``.
.. method:: process_rhs(compiler, connection)
Behaves the same way as :meth:`process_lhs`, for the right-hand side.
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