Fix columns got dirty after copy and modify inner values.

Author: WaVEV

django_mongodb/query.py

@@ -121,12 +121,11 @@ def join(self, compiler, connection):
                 columns.append((expr.rhs, None))
         replacements = {}
         for col, parent_pos in columns:
-            column_target = col.copy()
-            if column_target.alias == self.table_name:
-                column_target.alias = compiler.collection_name
-            else:
-                column_target.target.db_column = f"{parent_template}{parent_pos}"
-                column_target.target.set_attributes_from_name(f"{parent_template}{len(lhs_fields)}")
+            # 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:
+                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
         extra_condition = [extra.replace_expressions(replacements).as_mql(compiler, connection)]
     else: