# Django 6.0.5 documentation

# Lookup API reference

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 Making queries; to learn how to create new lookups, see How to write custom lookups.

The lookup API has two components: a `RegisterLookupMixin` class that registers lookups, and the 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:

- `Lookup`: to lookup a field (e.g. the `exact` of `field_name__exact`)

- `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`.

## Registration API

Django uses `RegisterLookupMixin` to give a class the interface to register lookups on itself or its instances. The two prominent examples are `Field`, the base class of all model fields, and `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 or class instance. For example:

        DateField.register_lookup(YearExact)
        User._meta.get_field("date_joined").register_lookup(MonthExact)

    will register `YearExact` lookup on `DateField` and `MonthExact` lookup on the `User.date_joined` (you can use Field Access API to retrieve a single field instance). It overrides a lookup that already exists with the same name. Lookups registered on field instances take precedence over the lookups registered on classes. `lookup_name` will be used for this lookup if provided, otherwise `lookup.lookup_name` will be used.

  get_lookup(lookup_name)

    Returns the `Lookup` named `lookup_name` registered in the class or class instance depending on what calls it. The default implementation looks recursively on all parent classes and checks if any has a registered lookup named `lookup_name`, returning the first match. Instance lookups would override any class lookups with the same `lookup_name`.

  get_lookups()

    Returns a dictionary of each lookup name registered in the class or class instance mapped to the `Lookup` class.

  get_transform(transform_name)

    Returns a `Transform` named `transform_name` registered in the class or class instance. 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. `Lookup` and `Transform` naturally follow this API.

## 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:

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 `Func.as_sql()` for example usage.

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.

get_lookup(lookup_name)

  Must return the lookup named `lookup_name`. For instance, by returning `self.output_field.get_lookup(lookup_name)`.

get_transform(transform_name)

  Must return the lookup named `transform_name`. For instance, by returning `self.output_field.get_transform(transform_name)`.

output_field

  Defines the type of class returned by the `get_lookup()` method. It must be a `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 `<expression>__<transformation>` (e.g. `date__year`).

  This class follows the Query Expression API, which implies that you can use `<expression>__<transform1>__<transform2>`. It’s a specialized 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.

  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 How to write custom lookups.

  lhs

    The left-hand side - what is being transformed. It must follow the Query Expression API.

  lookup_name

    The name of the lookup, used for identifying it on parsing query expressions. It cannot contain the string `"__"`.

  output_field

    Defines the class this transformation outputs. It must be a `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, `lhs`; a right-hand side, `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 primary notation to use a lookup in an expression is `<lhs>__<lookup_name>=<rhs>`. Lookups can also be used directly in `QuerySet` filters:

      Book.objects.filter(LessThan(F("word_count"), 7500))

  …or annotations:

      Book.objects.annotate(is_short_story=LessThan(F("word_count"), 7500))

  lhs

    The left-hand side - what is being looked up. The object typically follows the Query Expression API. It may also be a plain value.

  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`.

  lookup_name

    The name of this lookup, used to identify it on parsing query expressions. It cannot contain the string `"__"`.

  prepare_rhs

    Defaults to `True`. When `rhs` is a plain value, `prepare_rhs` determines whether it should be prepared for use as a parameter in a query. In order to do so, `lhs.output_field.get_prep_value()` is called if defined, or `rhs` is wrapped in `Value()` otherwise.

  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`.

  process_rhs(compiler, connection)

    Behaves the same way as `process_lhs()`, for the right-hand side.