==================== 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 `, 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 :class:`Transform`, the base class of all Django transforms. .. 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. .. 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_lookups() Returns a dictionary of each lookup name registered in the class mapped to the :class:`Lookup` class. .. 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 `. :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(compiler, connection) Generates the SQL fragment for the expression. Returns a tuple ``(sql, params)``, where ``sql`` is the SQL string, and ``params`` is the list or tuple of query parameters. 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. Custom keyword arguments may be defined on this method if it's likely that ``as_vendorname()`` methods or subclasses will need to supply data to override the generation of the SQL string. See :meth:`Func.as_sql` for example usage. .. method:: as_vendorname(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 a lookup expression is ``__`` (e.g. ``date__year``). This class follows the :ref:`Query Expression API `, which implies that you can use ``____``. It's a specialized :ref:`Func() expression ` that only accepts one argument. It can also be used on the right hand side of a filter or directly as an annotation. .. 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 `. .. 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 ``__=``. This class acts as a query expression, but, since it has ``=`` on its construction, lookups must always be 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 `. .. 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.