1  ---
  2  title: "Retrievers"
  3  id: experimental-retrievers-api
  4  description: "Sweep through Document Stores and return a set of candidate documents that are relevant to the query."
  5  slug: "/experimental-retrievers-api"
  6  ---
  7  
  8  <a id="haystack_experimental.components.retrievers.chat_message_retriever"></a>
  9  
 10  ## Module haystack\_experimental.components.retrievers.chat\_message\_retriever
 11  
 12  <a id="haystack_experimental.components.retrievers.chat_message_retriever.ChatMessageRetriever"></a>
 13  
 14  ### ChatMessageRetriever
 15  
 16  Retrieves chat messages from the underlying ChatMessageStore.
 17  
 18  Usage example:
 19  ```python
 20  from haystack.dataclasses import ChatMessage
 21  from haystack_experimental.components.retrievers import ChatMessageRetriever
 22  from haystack_experimental.chat_message_stores.in_memory import InMemoryChatMessageStore
 23  
 24  messages = [
 25      ChatMessage.from_assistant("Hello, how can I help you?"),
 26      ChatMessage.from_user("Hi, I have a question about Python. What is a Protocol?"),
 27  ]
 28  
 29  message_store = InMemoryChatMessageStore()
 30  message_store.write_messages(chat_history_id="user_456_session_123", messages=messages)
 31  retriever = ChatMessageRetriever(message_store)
 32  
 33  result = retriever.run(chat_history_id="user_456_session_123")
 34  
 35  print(result["messages"])
 36  ```
 37  
 38  <a id="haystack_experimental.components.retrievers.chat_message_retriever.ChatMessageRetriever.__init__"></a>
 39  
 40  #### ChatMessageRetriever.\_\_init\_\_
 41  
 42  ```python
 43  def __init__(chat_message_store: ChatMessageStore, last_k: int | None = 10)
 44  ```
 45  
 46  Create the ChatMessageRetriever component.
 47  
 48  **Arguments**:
 49  
 50  - `chat_message_store`: An instance of a ChatMessageStore.
 51  - `last_k`: The number of last messages to retrieve. Defaults to 10 messages if not specified.
 52  
 53  <a id="haystack_experimental.components.retrievers.chat_message_retriever.ChatMessageRetriever.to_dict"></a>
 54  
 55  #### ChatMessageRetriever.to\_dict
 56  
 57  ```python
 58  def to_dict() -> dict[str, Any]
 59  ```
 60  
 61  Serializes the component to a dictionary.
 62  
 63  **Returns**:
 64  
 65  Dictionary with serialized data.
 66  
 67  <a id="haystack_experimental.components.retrievers.chat_message_retriever.ChatMessageRetriever.from_dict"></a>
 68  
 69  #### ChatMessageRetriever.from\_dict
 70  
 71  ```python
 72  @classmethod
 73  def from_dict(cls, data: dict[str, Any]) -> "ChatMessageRetriever"
 74  ```
 75  
 76  Deserializes the component from a dictionary.
 77  
 78  **Arguments**:
 79  
 80  - `data`: The dictionary to deserialize from.
 81  
 82  **Returns**:
 83  
 84  The deserialized component.
 85  
 86  <a id="haystack_experimental.components.retrievers.chat_message_retriever.ChatMessageRetriever.run"></a>
 87  
 88  #### ChatMessageRetriever.run
 89  
 90  ```python
 91  @component.output_types(messages=list[ChatMessage])
 92  def run(
 93      chat_history_id: str,
 94      *,
 95      last_k: int | None = None,
 96      current_messages: list[ChatMessage] | None = None
 97  ) -> dict[str, list[ChatMessage]]
 98  ```
 99  
100  Run the ChatMessageRetriever
101  
102  **Arguments**:
103  
104  - `chat_history_id`: A unique identifier for the chat session or conversation whose messages should be retrieved.
105  Each `chat_history_id` corresponds to a distinct chat history stored in the underlying ChatMessageStore.
106  For example, use a session ID or conversation ID to isolate messages from different chat sessions.
107  - `last_k`: The number of last messages to retrieve. This parameter takes precedence over the last_k
108  parameter passed to the ChatMessageRetriever constructor. If unspecified, the last_k parameter passed
109  to the constructor will be used.
110  - `current_messages`: A list of incoming chat messages to combine with the retrieved messages. System messages from this list
111  are prepended before the retrieved history, while all other messages (e.g., user messages) are appended
112  after. This is useful for including new conversational context alongside stored history so the output
113  can be directly used as input to a ChatGenerator or an Agent. If not provided, only the stored messages
114  will be returned.
115  
116  **Raises**:
117  
118  - `ValueError`: If last_k is not None and is less than 0.
119  
120  **Returns**:
121  
122  A dictionary with the following key:
123  - `messages` - The retrieved chat messages combined with any provided current messages.