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:
A lookup expression consists of three parts:
Book.objects.filter(author__best_friends__first_name...);__lower__first3chars__reversed);__icontains) that, if omitted, defaults to __exact.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.
lookups.RegisterLookupMixin¶A mixin that implements the lookup API on a class.
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.
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 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).
Transform reference¶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 "__".
Lookup reference¶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 acts as a query expression, but, since it has =<rhs> on its
construction, lookups must always be 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.
Jul 27, 2022