Refactor.

Author: WaVEV

django_mongodb/query.py

@@ -110,16 +110,20 @@ def join(self, compiler, connection):
     if extra:
         columns = []
         for expr in extra.leaves():
-            if hasattr(expr, "lhs") and isinstance(expr.lhs, Col):
-                columns.append((expr.lhs, len(lhs_fields)))
-                lhs_fields.append(expr.lhs.as_mql(compiler, connection))
-            if hasattr(expr, "rhs") and isinstance(expr.rhs, Col):
-                columns.append((expr.rhs, None))
+            for hand_side in ["lhs", "rhs"]:
+                hand_side_value = getattr(expr, hand_side, None)
+                if isinstance(hand_side_value, Col):
+                    if hand_side_value.alias != self.table_name:
+                        pos = len(lhs_fields)
+                        lhs_fields.append(expr.lhs.as_mql(compiler, connection))
+                    else:
+                        pos = None
+                    columns.append((hand_side_value, pos))
         replacements = {}
         for col, parent_pos in columns:
             # Make all columns in as main collection columns.
             column_target = Col(compiler.collection_name, expr.output_field.__class__())
-            if column_target.alias != self.table_name:
+            if parent_pos is not None:
                 column_target.target.db_column = f"${parent_template}{parent_pos}"
                 column_target.target.set_attributes_from_name(f"${parent_template}{parent_pos}")
             replacements[col] = column_target