Fix unintended overwrite of column.target.

Author: WaVEV

django_mongodb_backend/query.py

@@ -126,7 +126,7 @@ def extra_where(self, compiler, connection):  # noqa: ARG001
 def join(self, compiler, connection, pushed_expressions=None):
     def _get_reroot_replacements(expressions):
         if not expressions:
-            return []
+            return None
         columns = []
         for expr in expressions:
             # Determine whether the column needs to be transformed or rerouted
@@ -146,7 +146,9 @@ def _get_reroot_replacements(expressions):
         # based on their rerouted positions in the join pipeline.
         replacements = {}
         for col, parent_pos in columns:
-            column_target = Col(compiler.collection_name, col.target, col.output_field)
+            target = col.target.clone()
+            target.remote_field = col.target.remote_field
+            column_target = Col(compiler.collection_name, target)
             if parent_pos is not None:
                 target_col = f"${parent_template}{parent_pos}"
                 column_target.target.db_column = target_col