Preload only first nested association's association

Hello all,

I’m doing a DB call for the index page of a WhatsApp-style chat app.

I have ChatsUsers → Chats → Messages.

I need to get the list of the user’s chats ordered by recency of the last_message, and just for the first Chat preload all of its Messages.

See below for reference.

Here’s the DB call I’m trying to make

ChatUser
    |> where(user_id: ^user_id)
    |> join(:left, [cu], c in assoc(cu, :chat)) 
    |> order_by([cu, c], [{:desc, c.last_message}]) 
    |> preload([cu, c], chat: c)
    |> Repo.all()

This will order all the ChatsUsers based on the descending order of its chat.last_message.

But now I need to preload the messages to only the first ChatsUsers’s chat.

Any tips appreciated!