feat: ChatPromptBuilder copies entire ChatMessage rather than copying content field only (#8317)

* Initial implementation of ChatMessage copy and deepcopy

* Add reno release note

* Satisfy hawkeye

* Remove copy and deepcopy, no need to complicate things

* Add new reno note

* Add unit test

haystack/components/builders/chat_prompt_builder.py

@@ -2,6 +2,7 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
+from copy import deepcopy
 from typing import Any, Dict, List, Optional, Set
 
 from jinja2 import meta
@@ -194,11 +195,9 @@ class ChatPromptBuilder:
 
                 compiled_template = self._env.from_string(message.content)
                 rendered_content = compiled_template.render(template_variables_combined)
-                rendered_message = (
-                    ChatMessage.from_user(rendered_content)
-                    if message.is_from(ChatRole.USER)
-                    else ChatMessage.from_system(rendered_content)
-                )
+                # deep copy the message to avoid modifying the original message
+                rendered_message: ChatMessage = deepcopy(message)
+                rendered_message.content = rendered_content
                 processed_messages.append(rendered_message)
             else:
                 processed_messages.append(message)

releasenotes/notes/adapt-chat-promp-builder-message-copy-6aa4de39363ffbf2.yaml

@@ -0,0 +1,4 @@
+---
+enhancements:
+  - |
+    Adapts how ChatPromptBuilder creates ChatMessages. Messages are deep copied to ensure all meta fields are copied correctly.

test/components/builders/test_chat_prompt_builder.py

@@ -5,7 +5,7 @@ import pytest
 from haystack.components.builders.chat_prompt_builder import ChatPromptBuilder
 from haystack import component
 from haystack.core.pipeline.pipeline import Pipeline
-from haystack.dataclasses.chat_message import ChatMessage
+from haystack.dataclasses.chat_message import ChatMessage, ChatRole
 from haystack.dataclasses.document import Document
 
 
@@ -194,6 +194,17 @@ class TestChatPromptBuilder:
 
         assert builder.run(template, name="John", var1="Big") == expected_result
 
+    def test_run_with_meta(self):
+        """
+        Test that the ChatPromptBuilder correctly handles meta data.
+        It should render the message and copy the meta data from the original message.
+        """
+        m = ChatMessage(content="This is a {{ variable }}", role=ChatRole.USER, name=None, meta={"test": "test"})
+        builder = ChatPromptBuilder(template=[m])
+        res = builder.run(variable="test")
+        res_msg = ChatMessage(content="This is a test", role=ChatRole.USER, name=None, meta={"test": "test"})
+        assert res == {"prompt": [res_msg]}
+
     def test_run_with_invalid_template(self):
         builder = ChatPromptBuilder()