fix queries where the right-hand side is a field

Author: timgraham

django_mongodb/features.py

@@ -138,8 +138,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
             "lookup.tests.LookupQueryingTests.test_filter_exists_lhs",
             # QuerySet.alias() doesn't work.
             "lookup.tests.LookupQueryingTests.test_alias",
-            # Comparing two fields doesn't work.
-            "lookup.tests.LookupQueryingTests.test_annotate_field_greater_than_field",
             # Value() not supported.
             "lookup.tests.LookupQueryingTests.test_annotate_literal_greater_than_field",
             "lookup.tests.LookupQueryingTests.test_annotate_value_greater_than_value",

django_mongodb/query_utils.py

@@ -16,6 +16,9 @@ def process_lhs(node, compiler, connection, bare_column_ref=False):
 
 
 def process_rhs(node, compiler, connection):
+    rhs = node.rhs
+    if hasattr(rhs, "as_mql"):
+        return rhs.as_mql(compiler, connection)
     _, value = node.process_rhs(compiler, connection)
     lookup_name = node.lookup_name
     # Undo Lookup.get_db_prep_lookup() putting params in a list.