Minor cleanup of Lookup API

django/db/models/lookups.py

@@ -1,6 +1,5 @@
 from copy import copy
 
-from django.core.exceptions import FieldError
 from django.conf import settings
 from django.utils import timezone
 from django.utils.functional import cached_property
@@ -29,19 +28,11 @@ class Extract(object):
 
 class Lookup(object):
     lookup_name = None
-    extract_class = None
 
     def __init__(self, lhs, rhs):
         self.lhs, self.rhs = lhs, rhs
-        if rhs is None:
-            if not self.extract_class:
-                raise FieldError("Lookup '%s' doesn't support nesting." % self.lookup_name)
-        else:
         self.rhs = self.get_prep_lookup()
 
-    def get_extract(self):
-        return self.extract_class(self.lhs)
-
     def get_prep_lookup(self):
         return self.lhs.output_type.get_prep_lookup(self.lookup_name, self.rhs)
 

docs/ref/models/lookups.txt

@@ -93,16 +93,9 @@ The process_rhs method is used to convert the right hand side into query string.
 The rhs is the value given in the filter clause. It can be a raw value to
 compare agains, a F() reference to another field or even a QuerySet.
 
-.. method:: get_extract()
-
-The get_extract method is used in nested lookups. It must return an Extract instance.
-
-.. classattribute:: Lookup.extract_class
-
-The default implementation of get_extract() will return an instance of extract_class.
-
-In addition there are some private methods - that is, implementing just the above
-mentioned attributes and methods is not enough, you must subclass Lookup instead.
+In addition the Lookup class has some private methods - that is, implementing
+just the above mentioned attributes and methods is not enough, instead you
+should subclass Lookup.
 
 The Extract class
 ~~~~~~~~~~~~~~~~~
@@ -112,7 +105,7 @@ For example you could have an Extract that procudes modulo 3 of the given value.
 In SQL this would be something like "author"."age" % 3.
 
 Extracts are used in nested lookups. The Extract class must implement the query
-part interface.
+part interface. In addition the Extract should must lookup_name attribute.
 
 A simple Lookup example
 ~~~~~~~~~~~~~~~~~~~~~~~