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Thanks Simon Charette and Mariusz Felisiak for reviews and mentoring this Google Summer of Code 2022 project.
235 lines
9.3 KiB
Plaintext
235 lines
9.3 KiB
Plaintext
====================
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Lookup API reference
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====================
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.. module:: django.db.models.lookups
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:synopsis: Lookups API
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.. currentmodule:: django.db.models
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This document has the API references of lookups, the Django API for building
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the ``WHERE`` clause of a database query. To learn how to *use* lookups, see
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:doc:`/topics/db/queries`; to learn how to *create* new lookups, see
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:doc:`/howto/custom-lookups`.
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The lookup API has two components: a :class:`~lookups.RegisterLookupMixin` class
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that registers lookups, and the :ref:`Query Expression API <query-expression>`, a
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set of methods that a class has to implement to be registrable as a lookup.
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Django has two base classes that follow the query expression API and from where
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all Django builtin lookups are derived:
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* :class:`Lookup`: to lookup a field (e.g. the ``exact`` of ``field_name__exact``)
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* :class:`Transform`: to transform a field
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A lookup expression consists of three parts:
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* Fields part (e.g. ``Book.objects.filter(author__best_friends__first_name...``);
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* Transforms part (may be omitted) (e.g. ``__lower__first3chars__reversed``);
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* A lookup (e.g. ``__icontains``) that, if omitted, defaults to ``__exact``.
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.. _lookup-registration-api:
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Registration API
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================
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Django uses :class:`~lookups.RegisterLookupMixin` to give a class the interface to
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register lookups on itself or its instances. The two prominent examples are
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:class:`~django.db.models.Field`, the base class of all model fields, and
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:class:`Transform`, the base class of all Django transforms.
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.. class:: lookups.RegisterLookupMixin
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A mixin that implements the lookup API on a class.
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.. classmethod:: register_lookup(lookup, lookup_name=None)
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Registers a new lookup in the class or class instance. For example::
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DateField.register_lookup(YearExact)
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User._meta.get_field('date_joined').register_lookup(MonthExact)
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will register ``YearExact`` lookup on ``DateField`` and ``MonthExact``
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lookup on the ``User.date_joined`` (you can use :ref:`Field Access API
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<model-meta-field-api>` to retrieve a single field instance). It
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overrides a lookup that already exists with the same name. Lookups
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registered on field instances take precedence over the lookups
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registered on classes. ``lookup_name`` will be used for this lookup if
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provided, otherwise ``lookup.lookup_name`` will be used.
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.. method:: get_lookup(lookup_name)
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Returns the :class:`Lookup` named ``lookup_name`` registered in the
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class or class instance depending on what calls it. The default
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implementation looks recursively on all parent classes and checks if
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any has a registered lookup named ``lookup_name``, returning the first
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match. Instance lookups would override any class lookups with the same
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``lookup_name``.
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.. method:: get_lookups()
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Returns a dictionary of each lookup name registered in the class or
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class instance mapped to the :class:`Lookup` class.
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.. method:: get_transform(transform_name)
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Returns a :class:`Transform` named ``transform_name`` registered in the
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class or class instance. The default implementation looks recursively
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on all parent classes to check if any has the registered transform
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named ``transform_name``, returning the first match.
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For a class to be a lookup, it must follow the :ref:`Query Expression API
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<query-expression>`. :class:`~Lookup` and :class:`~Transform` naturally
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follow this API.
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.. versionchanged:: 4.2
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Support for registering lookups on :class:`~django.db.models.Field`
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instances was added.
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.. _query-expression:
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The Query Expression API
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========================
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The query expression API is a common set of methods that classes define to be
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usable in query expressions to translate themselves into SQL expressions. Direct
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field references, aggregates, and ``Transform`` are examples that follow this
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API. A class is said to follow the query expression API when it implements the
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following methods:
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.. method:: as_sql(compiler, connection)
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Generates the SQL fragment for the expression. Returns a tuple
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``(sql, params)``, where ``sql`` is the SQL string, and ``params`` is the
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list or tuple of query parameters. The ``compiler`` is an ``SQLCompiler``
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object, which has a ``compile()`` method that can be used to compile other
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expressions. The ``connection`` is the connection used to execute the
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query.
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Calling ``expression.as_sql()`` is usually incorrect - instead
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``compiler.compile(expression)`` should be used. The ``compiler.compile()``
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method will take care of calling vendor-specific methods of the expression.
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Custom keyword arguments may be defined on this method if it's likely that
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``as_vendorname()`` methods or subclasses will need to supply data to
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override the generation of the SQL string. See :meth:`Func.as_sql` for
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example usage.
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.. method:: as_vendorname(compiler, connection)
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Works like ``as_sql()`` method. When an expression is compiled by
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``compiler.compile()``, Django will first try to call ``as_vendorname()``,
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where ``vendorname`` is the vendor name of the backend used for executing
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the query. The ``vendorname`` is one of ``postgresql``, ``oracle``,
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``sqlite``, or ``mysql`` for Django's built-in backends.
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.. method:: get_lookup(lookup_name)
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Must return the lookup named ``lookup_name``. For instance, by returning
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``self.output_field.get_lookup(lookup_name)``.
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.. method:: get_transform(transform_name)
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Must return the lookup named ``transform_name``. For instance, by returning
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``self.output_field.get_transform(transform_name)``.
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.. attribute:: output_field
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Defines the type of class returned by the ``get_lookup()`` method. It must
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be a :class:`~django.db.models.Field` instance.
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``Transform`` reference
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=======================
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.. class:: Transform
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A ``Transform`` is a generic class to implement field transformations. A
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prominent example is ``__year`` that transforms a ``DateField`` into a
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``IntegerField``.
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The notation to use a ``Transform`` in a lookup expression is
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``<expression>__<transformation>`` (e.g. ``date__year``).
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This class follows the :ref:`Query Expression API <query-expression>`, which
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implies that you can use ``<expression>__<transform1>__<transform2>``. It's
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a specialized :ref:`Func() expression <func-expressions>` that only accepts
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one argument. It can also be used on the right hand side of a filter or
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directly as an annotation.
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.. attribute:: bilateral
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A boolean indicating whether this transformation should apply to both
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``lhs`` and ``rhs``. Bilateral transformations will be applied to ``rhs`` in
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the same order as they appear in the lookup expression. By default it is set
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to ``False``. For example usage, see :doc:`/howto/custom-lookups`.
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.. attribute:: lhs
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The left-hand side - what is being transformed. It must follow the
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:ref:`Query Expression API <query-expression>`.
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.. attribute:: lookup_name
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The name of the lookup, used for identifying it on parsing query
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expressions. It cannot contain the string ``"__"``.
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.. attribute:: output_field
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Defines the class this transformation outputs. It must be a
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:class:`~django.db.models.Field` instance. By default is the same as
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its ``lhs.output_field``.
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``Lookup`` reference
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====================
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.. class:: Lookup
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A ``Lookup`` is a generic class to implement lookups. A lookup is a query
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expression with a left-hand side, :attr:`lhs`; a right-hand side,
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:attr:`rhs`; and a ``lookup_name`` that is used to produce a boolean
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comparison between ``lhs`` and ``rhs`` such as ``lhs in rhs`` or
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``lhs > rhs``.
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The primary notation to use a lookup in an expression is
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``<lhs>__<lookup_name>=<rhs>``. Lookups can also be used directly in
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``QuerySet`` filters::
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Book.objects.filter(LessThan(F('word_count'), 7500))
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…or annotations::
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Book.objects.annotate(is_short_story=LessThan(F('word_count'), 7500))
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.. attribute:: lhs
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The left-hand side - what is being looked up. The object typically
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follows the :ref:`Query Expression API <query-expression>`. It may also
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be a plain value.
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.. attribute:: rhs
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The right-hand side - what ``lhs`` is being compared against. It can be
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a plain value, or something that compiles into SQL, typically an
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``F()`` object or a ``QuerySet``.
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.. attribute:: lookup_name
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The name of this lookup, used to identify it on parsing query
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expressions. It cannot contain the string ``"__"``.
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.. method:: process_lhs(compiler, connection, lhs=None)
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Returns a tuple ``(lhs_string, lhs_params)``, as returned by
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``compiler.compile(lhs)``. This method can be overridden to tune how
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the ``lhs`` is processed.
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``compiler`` is an ``SQLCompiler`` object, to be used like
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``compiler.compile(lhs)`` for compiling ``lhs``. The ``connection``
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can be used for compiling vendor specific SQL. If ``lhs`` is not
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``None``, use it as the processed ``lhs`` instead of ``self.lhs``.
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.. method:: process_rhs(compiler, connection)
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Behaves the same way as :meth:`process_lhs`, for the right-hand side.
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