Making func expressions into Transforms

func_as_transform.diff

diff --git django/db/models/functions.py django/db/models/functions.py
index c3dea1f..30046ea 100644
--- django/db/models/functions.py
+++ django/db/models/functions.py
@@ -2,7 +2,7 @@
 Classes that represent database functions.
 """
 from django.db.models.expressions import Func, Value
-from django.db.models import IntegerField
+from django.db.models import IntegerField, Transform


 class Coalesce(Func):
@@ -69,11 +69,14 @@ class Concat(Func):
         return ConcatPair(expressions[0], self._paired(expressions[1:]))


-class Length(Func):
+class Length(Func, Transform):
     """Returns the number of characters in the expression"""
     function = 'LENGTH'
+    lookup_name = 'length'

-    def __init__(self, expression, **extra):
+    def __init__(self, expression, lookups=None, **extra):
+        if lookups is not None:
+            self.init_lookups = lookups[:]
         output_field = extra.pop('output_field', IntegerField())
         super(Length, self).__init__(expression, output_field=output_field, **extra)

@@ -81,6 +84,10 @@ class Length(Func):
         self.function = 'CHAR_LENGTH'
         return super(Length, self).as_sql(compiler, connection)

+    @property
+    def lhs(self):
+        return self.get_source_expressions()[0]
+

 class Lower(Func):
     function = 'LOWER'
diff --git tests/functions/tests.py tests/functions/tests.py
index 32e0970..0fcd59e 100644
--- tests/functions/tests.py
+++ tests/functions/tests.py
@@ -157,6 +157,20 @@ class FunctionTests(TestCase):
             authors.filter(alias_length__lte=Length('name')).count(),
             1)

+    def test_length_transform(self):
+        try:
+            CharField.register_lookup(Length)
+            Author.objects.create(name='John Smith', alias='smithj')
+            Author.objects.create(name='Rhonda')
+            authors = Author.objects.filter(name__length__gt=7)
+            self.assertQuerysetEqual(
+                authors.order_by('name'), [
+                    'John Smith'
+                ],
+                lambda a: a.name)
+        finally:
+            CharField._unregister_lookup(Length)
+
     def test_substr(self):
         Author.objects.create(name='John Smith', alias='smithj')
         Author.objects.create(name='Rhonda')