mirror of
https://github.com/django/django.git
synced 2025-08-03 10:34:04 +00:00
Fixed #22812 -- Refactored lookup API documentation.
Thanks Anssi and Tim for reviews.
This commit is contained in:
Jorge C. Leitão
Tim Graham
parent
503e59c9b0
commit
8780849da0
8 changed files with 223 additions and 153 deletions
|
|
@ -1,407 +0,0 @@
|
|||
==============
|
||||
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 ``__``.
|
||||
|
||||
We then need to define the ``as_sql`` method. This takes a ``SQLCompiler``
|
||||
object, called ``qn``, and the active database connection. ``SQLCompiler``
|
||||
objects are not documented, but the only thing we need to know about them is
|
||||
that they have a ``compile()`` method which returns a tuple containing a SQL
|
||||
string, and the parameters to be interpolated into that string. In most cases,
|
||||
you don't need to use it directly and can pass it on to ``process_lhs()`` and
|
||||
``process_rhs()``.
|
||||
|
||||
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 using the ``qn`` object described before. These methods return
|
||||
tuples containing some SQL and the parameters to be interpolated into that SQL,
|
||||
just as we need to return from our ``as_sql`` method. 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 exceed 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 queries 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_field`` 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_field = 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 backends 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 implementations 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 behavior 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 the same ``lookup_name``.
|
||||
|
||||
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``.
|
||||
|
||||
How Django determines the lookups and transforms which are used
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
In some cases you may which to dynamically change which ``Transform`` or
|
||||
``Lookup`` is returned based on the name passed in, rather than fixing it. As
|
||||
an example, you could have a field which stores coordinates or an arbitrary
|
||||
dimension, and wish to allow a syntax like ``.filter(coords__x7=4)`` to return
|
||||
the objects where the 7th coordinate has value 4. In order to do this, you
|
||||
would override ``get_lookup`` with something like::
|
||||
|
||||
class CoordinatesField(Field):
|
||||
def get_lookup(self, lookup_name):
|
||||
if lookup_name.startswith('x'):
|
||||
try:
|
||||
dimension = int(lookup_name[1:])
|
||||
except ValueError:
|
||||
pass
|
||||
finally:
|
||||
return get_coordinate_lookup(dimension)
|
||||
return super(CoordinatesField, self).get_lookup(lookup_name)
|
||||
|
||||
You would then define ``get_coordinate_lookup`` appropriately to return a
|
||||
``Lookup`` subclass which handles the relevant value of ``dimension``.
|
||||
|
||||
There is a similarly named method called ``get_transform()``. ``get_lookup()``
|
||||
should always return a ``Lookup`` subclass, and ``get_transform()`` a
|
||||
``Transform`` subclass. It is important to remember that ``Transform``
|
||||
objects can be further filtered on, and ``Lookup`` objects cannot.
|
||||
|
||||
When filtering, if there is only one lookup name remaining to be resolved, we
|
||||
will look for a ``Lookup``. If there are multiple names, it will look for a
|
||||
``Transform``. In the situation where there is only one name and a ``Lookup``
|
||||
is not found, we look for a ``Transform`` and then the ``exact`` lookup on that
|
||||
``Transform``. All call sequences always end with a ``Lookup``. To clarify:
|
||||
|
||||
- ``.filter(myfield__mylookup)`` will call ``myfield.get_lookup('mylookup')``.
|
||||
- ``.filter(myfield__mytransform__mylookup)`` will call
|
||||
``myfield.get_transform('mytransform')``, and then
|
||||
``mytransform.get_lookup('mylookup')``.
|
||||
- ``.filter(myfield__mytransform)`` will first call
|
||||
``myfield.get_lookup('mytransform')``, which will fail, so it will fall back
|
||||
to calling ``myfield.get_transform('mytransform')`` and then
|
||||
``mytransform.get_lookup('exact')``.
|
||||
|
||||
Lookups and transforms are registered using the same API - ``register_lookup``.
|
||||
|
||||
.. _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`` is a ``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() 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:: 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.
|
||||
|
||||
.. 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 behavior.
|
||||
|
||||
.. method:: get_transform(lookup_name)
|
||||
|
||||
The ``get_transform()`` method is used when a transform is needed rather
|
||||
than a lookup, or if a lookup is not found. This is a more complex
|
||||
situation which is useful when there arbitrary possible lookups for a
|
||||
field. Generally speaking, you will not need to override ``get_lookup()``
|
||||
or ``get_transform()``, and can use ``register_lookup()`` instead.
|
||||
|
||||
.. attribute:: output_field
|
||||
|
||||
The ``output_field`` attribute is used by the ``get_lookup()`` method to
|
||||
check for lookups. The ``output_field`` 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``). You can
|
||||
register a Lookup or a Transform using the same class method.
|
||||
|
||||
.. method:: get_lookup(lookup_name)
|
||||
|
||||
Django uses ``get_lookup(lookup_name)`` to fetch lookups. The
|
||||
implementation of ``get_lookup()`` looks for a subclass which is registered
|
||||
for the current class with the correct ``lookup_name``.
|
||||
|
||||
.. method:: get_transform(lookup_name)
|
||||
|
||||
Django uses ``get_transform(lookup_name)`` to fetch transforms. The
|
||||
implementation of ``get_transform()`` looks for a subclass which is registered
|
||||
for the current class with the correct ``transform_name``.
|
||||
|
||||
The lookup registration API is available for ``Transform`` and ``Field`` classes.
|
||||
|
|
@ -14,4 +14,4 @@ Model API reference. For introductory material, see :doc:`/topics/db/models`.
|
|||
instances
|
||||
querysets
|
||||
queries
|
||||
custom-lookups
|
||||
lookups
|
||||
|
|
|
|||
207
docs/ref/models/lookups.txt
Normal file
207
docs/ref/models/lookups.txt
Normal file
|
|
@ -0,0 +1,207 @@
|
|||
====================
|
||||
Lookup API reference
|
||||
====================
|
||||
|
||||
.. module:: django.db.models.lookups
|
||||
:synopsis: Lookups API
|
||||
|
||||
.. currentmodule:: django.db.models
|
||||
|
||||
.. versionadded:: 1.7
|
||||
|
||||
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 `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)
|
||||
|
||||
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.
|
||||
|
||||
.. 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 `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, qn, connection)
|
||||
|
||||
Responsible for producing the query string and parameters for the expression.
|
||||
The ``qn`` 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
|
||||
``qn.compile(expression)`` should be used. The ``qn.compile()`` method will
|
||||
take care of calling vendor-specific methods of the expression.
|
||||
|
||||
.. method:: as_vendorname(self, 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.
|
||||
|
||||
.. 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 `Query Expression API <query-expression>`_, which
|
||||
implies that you can use ``<expression>__<transform1>__<transform2>``.
|
||||
|
||||
.. attribute:: lhs
|
||||
|
||||
The left-hand side - what is being transformed. It must follow the
|
||||
`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``.
|
||||
|
||||
.. method:: as_sql
|
||||
|
||||
To be overridden; raises :exc:`NotImplementedError`.
|
||||
|
||||
.. method:: get_lookup(lookup_name)
|
||||
|
||||
Same as :meth:`~lookups.RegisterLookupMixin.get_lookup()`.
|
||||
|
||||
.. method:: get_transform(transform_name)
|
||||
|
||||
Same as :meth:`~lookups.RegisterLookupMixin.get_transform()`.
|
||||
|
||||
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 `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 `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(qn, connection[, lhs=None])
|
||||
|
||||
Returns a tuple ``(lhs_string, lhs_params)``, as returned by
|
||||
``qn.compile(lhs)``. This method can be overridden to tune how the
|
||||
``lhs`` is processed.
|
||||
|
||||
``qn`` is an ``SQLCompiler`` object, to be used like ``qn.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(qn, connection)
|
||||
|
||||
Behaves the same way as :meth:`process_lhs`, for the right-hand side.
|
||||
|
|
@ -2061,7 +2061,7 @@ For an introduction, see :ref:`models and database queries documentation
|
|||
<field-lookups-intro>`.
|
||||
|
||||
Django's inbuilt lookups are listed below. It is also possible to write
|
||||
:doc:`custom lookups </ref/models/custom-lookups>` for model fields.
|
||||
:doc:`custom lookups </howto/custom-lookups>` for model fields.
|
||||
|
||||
As a convenience when no lookup type is provided (like in
|
||||
``Entry.objects.get(id=14)``) the lookup type is assumed to be :lookup:`exact`.
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue