fix: ChatPromptBuilder Fails to JSON Serialize (#7849)

* implement serialization for chat messages and add tests

* implement serialization for ChatPromptBuilder and test it

* add reno

* solve mypy type error

* solve mypy type error

* remove flattening parameter in to_dict

* simplify to jus non-flat metadata

* try to fix linting issue

* solve format issues

* update test for ChatPromptBuilder

* remove unused import

---------

Co-authored-by: David S. Batista <dsbatista@gmail.com>

haystack/components/builders/chat_prompt_builder.py

@@ -6,7 +6,7 @@ from typing import Any, Dict, List, Optional, Set
 
 from jinja2 import Template, meta
 
-from haystack import component, logging
+from haystack import component, default_from_dict, default_to_dict, logging
 from haystack.dataclasses.chat_message import ChatMessage, ChatRole
 
 logger = logging.getLogger(__name__)
@@ -223,3 +223,36 @@ class ChatPromptBuilder:
                 f"Missing required input variables in ChatPromptBuilder: {missing_vars_str}. "
                 f"Required variables: {self.required_variables}. Provided variables: {provided_variables}."
             )
+
+    def to_dict(self) -> Dict[str, Any]:
+        """
+        Returns a dictionary representation of the component.
+
+        :returns:
+            Serialized dictionary representation of the component.
+        """
+        if self.template is not None:
+            template = [m.to_dict() for m in self.template]
+        else:
+            template = None
+
+        return default_to_dict(
+            self, template=template, variables=self._variables, required_variables=self._required_variables
+        )
+
+    @classmethod
+    def from_dict(cls, data: Dict[str, Any]) -> "ChatPromptBuilder":
+        """
+        Deserialize this component from a dictionary.
+
+        :param data:
+            The dictionary to deserialize and create the component.
+
+        :returns:
+            The deserialized component.
+        """
+        init_parameters = data["init_parameters"]
+        template = init_parameters.get("template", [])
+        init_parameters["template"] = [ChatMessage.from_dict(d) for d in template]
+
+        return default_from_dict(cls, data)

haystack/dataclasses/chat_message.py

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
-from dataclasses import dataclass, field
+from dataclasses import asdict, dataclass, field
 from enum import Enum
 from typing import Any, Dict, Optional
 
@@ -99,3 +99,29 @@ class ChatMessage:
         :returns: A new ChatMessage instance.
         """
         return cls(content, ChatRole.FUNCTION, name)
+
+    def to_dict(self) -> Dict[str, Any]:
+        """
+        Converts ChatMessage into a dictionary.
+
+        :returns:
+            Serialized version of the object.
+        """
+        data = asdict(self)
+        data["role"] = self.role.value
+
+        return data
+
+    @classmethod
+    def from_dict(cls, data: Dict[str, Any]) -> "ChatMessage":
+        """
+        Creates a new ChatMessage object from a dictionary.
+
+        :param data:
+            The dictionary to build the ChatMessage object.
+        :returns:
+            The created object.
+        """
+        data["role"] = ChatRole(data["role"])
+
+        return cls(**data)

releasenotes/notes/fix-chat-prompt-builder-serialization-345605337ca2584e.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Solve serialization bug on 'ChatPromptBuilder' by creating 'to_dict' and 'from_dict' methods on 'ChatMessage' and 'ChatPromptBuilder'.

test/components/builders/test_chat_prompt_builder.py

@@ -494,3 +494,44 @@ class TestChatPromptBuilderDynamic:
                 ]
             }
         }
+
+    def test_to_dict(self):
+        component = ChatPromptBuilder(
+            template=[ChatMessage.from_user("text and {var}"), ChatMessage.from_assistant("content {required_var}")],
+            variables=["var", "required_var"],
+            required_variables=["required_var"],
+        )
+
+        assert component.to_dict() == {
+            "type": "haystack.components.builders.chat_prompt_builder.ChatPromptBuilder",
+            "init_parameters": {
+                "template": [
+                    {"content": "text and {var}", "role": "user", "name": None, "meta": {}},
+                    {"content": "content {required_var}", "role": "assistant", "name": None, "meta": {}},
+                ],
+                "variables": ["var", "required_var"],
+                "required_variables": ["required_var"],
+            },
+        }
+
+    def test_from_dict(self):
+        component = ChatPromptBuilder.from_dict(
+            data={
+                "type": "haystack.components.builders.chat_prompt_builder.ChatPromptBuilder",
+                "init_parameters": {
+                    "template": [
+                        {"content": "text and {var}", "role": "user", "name": None, "meta": {}},
+                        {"content": "content {required_var}", "role": "assistant", "name": None, "meta": {}},
+                    ],
+                    "variables": ["var", "required_var"],
+                    "required_variables": ["required_var"],
+                },
+            }
+        )
+
+        assert component.template == [
+            ChatMessage.from_user("text and {var}"),
+            ChatMessage.from_assistant("content {required_var}"),
+        ]
+        assert component._variables == ["var", "required_var"]
+        assert component._required_variables == ["required_var"]

test/dataclasses/test_chat_message.py

@@ -81,3 +81,22 @@ def test_apply_custom_chat_templating_on_chat_message():
         formatted_messages, chat_template=anthropic_template, tokenize=False
     )
     assert tokenized_messages == "You are good assistant\nHuman: I have a question\nAssistant:"
+
+
+def test_to_dict():
+    message = ChatMessage.from_user("content")
+    message.meta["some"] = "some"
+
+    assert message.to_dict() == {"content": "content", "role": "user", "name": None, "meta": {"some": "some"}}
+
+
+def test_from_dict():
+    assert ChatMessage.from_dict(data={"content": "text", "role": "user", "name": None}) == ChatMessage(
+        content="text", role=ChatRole("user"), name=None, meta={}
+    )
+
+
+def test_from_dict_with_meta():
+    assert ChatMessage.from_dict(
+        data={"content": "text", "role": "user", "name": None, "meta": {"something": "something"}}
+    ) == ChatMessage(content="text", role=ChatRole("user"), name=None, meta={"something": "something"})