Fixed #21863 -- supplemented get_lookup() with get_transform()

Also fixed #22124 -- Expanded explanation of exactly what is going on in
as_sql() methods.

3 files changed, 30 insertions, 18 deletions

diff --git a/django/db/models/lookups.py b/django/db/models/lookups.py
index 993f5a92ca..797f767a97 100644
--- a/django/db/models/lookups.py
+++ b/django/db/models/lookups.py
@@ -9,11 +9,11 @@ from django.utils.six.moves import xrange
 
 
 class RegisterLookupMixin(object):
-    def get_lookup(self, lookup_name):
+    def _get_lookup(self, lookup_name):
         try:
             return self.class_lookups[lookup_name]
         except KeyError:
-            # To allow for inheritance, check parent class class lookups.
+            # To allow for inheritance, check parent class' class_lookups.
             for parent in inspect.getmro(self.__class__):
                 if not 'class_lookups' in parent.__dict__:
                     continue
@@ -26,6 +26,18 @@ class RegisterLookupMixin(object):
             return self.output_type.get_lookup(lookup_name)
         return None
 
+    def get_lookup(self, lookup_name):
+        found = self._get_lookup(lookup_name)
+        if found is not None and not issubclass(found, Lookup):
+            return None
+        return found
+
+    def get_transform(self, lookup_name):
+        found = self._get_lookup(lookup_name)
+        if found is not None and not issubclass(found, Transform):
+            return None
+        return found
+
     @classmethod
     def register_lookup(cls, lookup):
         if not 'class_lookups' in cls.__dict__:
diff --git a/django/db/models/sql/datastructures.py b/django/db/models/sql/datastructures.py
index 421c3cd860..634edf2477 100644
--- a/django/db/models/sql/datastructures.py
+++ b/django/db/models/sql/datastructures.py
@@ -24,6 +24,9 @@ class Col(object):
     def get_lookup(self, name):
         return self.output_type.get_lookup(name)
 
+    def get_transform(self, name):
+        return self.output_type.get_transform(name)
+
     def prepare(self):
         return self
 
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 7bfdceb183..25ebe5598e 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1088,24 +1088,21 @@ class Query(object):
         lookups = lookups[:]
         while lookups:
             lookup = lookups[0]
-            next = lhs.get_lookup(lookup)
+            if len(lookups) == 1:
+                final_lookup = lhs.get_lookup(lookup)
+                if final_lookup:
+                    return final_lookup(lhs, rhs)
+                # We didn't find a lookup, so we are going to try get_transform
+                # + get_lookup('exact').
+                lookups.append('exact')
+            next = lhs.get_transform(lookup)
             if next:
-                if len(lookups) == 1:
-                    # This was the last lookup, so return value lookup.
-                    if issubclass(next, Transform):
-                        lookups.append('exact')
-                        lhs = next(lhs, lookups)
-                    else:
-                        return next(lhs, rhs)
-                else:
-                    lhs = next(lhs, lookups)
-            # A field's get_lookup() can return None to opt for backwards
-            # compatibility path.
-            elif len(lookups) > 2:
-                raise FieldError(
-                    "Unsupported lookup for field '%s'" % lhs.output_type.name)
+                lhs = next(lhs, lookups)
             else:
-                return None
+                raise FieldError(
+                    "Unsupported lookup '%s' for %s or join on the field not "
+                    "permitted." %
+                    (lookup, lhs.output_type.__class__.__name__))
             lookups = lookups[1:]
 
     def build_filter(self, filter_expr, branch_negated=False, current_negated=False,