haystack/docs-website/reference_versioned_docs/version-2.18/experiments-api/experimental_retrievers_api.md

title	id	description	slug
Retrievers	experimental-retrievers-api	Sweep through Document Stores and return a set of candidate documents that are relevant to the query.	/experimental-retrievers-api

Module haystack_experimental.components.retrievers.chat_message_retriever

ChatMessageRetriever

Retrieves chat messages from the underlying ChatMessageStore.

Usage example:

from haystack.dataclasses import ChatMessage
from haystack_experimental.components.retrievers import ChatMessageRetriever
from haystack_experimental.chat_message_stores.in_memory import InMemoryChatMessageStore

messages = [
    ChatMessage.from_assistant("Hello, how can I help you?"),
    ChatMessage.from_user("Hi, I have a question about Python. What is a Protocol?"),
]

message_store = InMemoryChatMessageStore()
message_store.write_messages(chat_history_id="user_456_session_123", messages=messages)
retriever = ChatMessageRetriever(message_store)

result = retriever.run(chat_history_id="user_456_session_123")

print(result["messages"])

ChatMessageRetriever.__init__

def __init__(chat_message_store: ChatMessageStore, last_k: Optional[int] = 10)

Create the ChatMessageRetriever component.

Arguments:

• chat_message_store: An instance of a ChatMessageStore.
• last_k: The number of last messages to retrieve. Defaults to 10 messages if not specified.

ChatMessageRetriever.to_dict

def to_dict() -> dict[str, Any]

Serializes the component to a dictionary.

Returns:

Dictionary with serialized data.

ChatMessageRetriever.from_dict

@classmethod
def from_dict(cls, data: dict[str, Any]) -> "ChatMessageRetriever"

Deserializes the component from a dictionary.

Arguments:

• data: The dictionary to deserialize from.

Returns:

The deserialized component.

ChatMessageRetriever.run

@component.output_types(messages=list[ChatMessage])
def run(
    chat_history_id: str,
    *,
    last_k: Optional[int] = None,
    current_messages: Optional[list[ChatMessage]] = None
) -> dict[str, list[ChatMessage]]

Run the ChatMessageRetriever

Arguments:

• chat_history_id: A unique identifier for the chat session or conversation whose messages should be retrieved. Each chat_history_id corresponds to a distinct chat history stored in the underlying ChatMessageStore. For example, use a session ID or conversation ID to isolate messages from different chat sessions.
• last_k: The number of last messages to retrieve. This parameter takes precedence over the last_k parameter passed to the ChatMessageRetriever constructor. If unspecified, the last_k parameter passed to the constructor will be used.
• current_messages: A list of incoming chat messages to combine with the retrieved messages. System messages from this list are prepended before the retrieved history, while all other messages (e.g., user messages) are appended after. This is useful for including new conversational context alongside stored history so the output can be directly used as input to a ChatGenerator or an Agent. If not provided, only the stored messages will be returned.

Raises:

• ValueError: If last_k is not None and is less than 0.

Returns:

A dictionary with the following key:

• messages - The retrieved chat messages combined with any provided current messages.