Avoid project field collision.

Author: WaVEV

django_mongodb/compiler.py

@@ -492,7 +492,8 @@ def get_project_fields(self, columns=None, ordering=None):
                     fields[alias] = 1
 
             for column, _ in ordering or []:
-                if column not in fields:
+                foreign_table = column.split(".", 1)[0] if "." in column else None
+                if column not in fields and foreign_table not in fields:
                     fields[column] = 1
 
         return fields