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 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. 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. 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.

get_lookup(lookup_name)

Returns the 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.

get_lookups()

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

get_transform(transform_name)

Returns a 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. 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)

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.

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 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 notation to use a lookup in an expression is <lhs>__<lookup_name>=<rhs>.

This class doesn’t follow the Query Expression API since it has =<rhs> on its construction: lookups are always the end of a lookup expression.

lhs

The left-hand side - what is being looked up. The object must follow the Query Expression API.

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 "__".

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.

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