Lookup API reference

New in Django 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 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 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.

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_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(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.

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.

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 an 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>.

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.

as_sql()

To be overridden; raises NotImplementedError.

get_lookup(lookup_name)

Same as get_lookup().

get_transform(transform_name)

Same as 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, 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(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.

process_rhs(qn, connection)

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

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