Does the story returned by Stories.create_story/1 have the user preloaded? If not and the parent liveview that gets notified of it then directly streams it, that would explain this error.

Edit 2:

I just realised you may be refering to existing records as the mounted records, and the preload may be failing on the updated stream when you are creating new records.

The below is what I use for updating messages in a stream.

Info() is specific to my app. Its something I use to share handle_info over multiple index routes, so treat it like handle_info.

Pass the message to a handle_info() then use the message id to get a preloaded version of the message, then insert it into the stream. Might be able to run repo.preload on the message in the params, can’t remember why I didn’t.

  def create_message(params, current_user, room) do
    if length(message_limit_reached(current_user, room)) >= 10 do
      {:ok, :rate_limit}
    else
      previous_message = get_previous_message(room)
      merge_flag       = if previous_message && previous_message.user_id === current_user.id, do: true, else: false

      Multi.new()
        |> Multi.insert(:message, %Message{user_id: current_user.id, room_id: room.id, merge: merge_flag} 
        |> Message.changeset(params))
        |> Repo.transaction()
        |> notify(room.id, :message_created)
      {:ok, :new_message}
    end

  def info(:message_created, %{message: message}, socket) do
    new_message = ecto_get!(Message, message.id, [:user])
    {:noreply, stream(socket, :messages, [new_message], at: 0)}
  end

  def ecto_get!(module, id, preload \\ []) do
    Repo.get!(module, id)
    |> Repo.preload(preload)
  end

Edit: The answer below is likely wrong, but still may be useful.

When you say existing records work, do you mean records you’ve seeded to the database?

Have you come accross IO.Inspect yet?

In your mount or heex add: IO.inspect(Stories.list_stories())

IO.inspect(Stories.list_stories())

or in the heex:

<%= IO.inspect(@streams.stories) %>

The above will output the inspect results to terminal and will consist of all returned stories by the list_stories function, and for every story returned, near the bottom of the Struct you will see all available preloads as lists.

To narrow down your issue the you can use inspect to see whether:

• No story preloads exist
• All story preloads exist
• Existing story preloads exist but new stories do not.

As an example you will get something like the below in the terminal from the inspect.

1 - If your preload isn’t set up, but an association exists it will return not loaded.
2 - If the preload is set up but no record exists then you will get nil or depending on whether its belongs_to or has_many.
3 - If the preload has been set up and the record exists, the struct will be returned

1 - children: #Ecto.Association.NotLoaded<association :children is not loaded>,
2 - comments: nil
3 - user: #Phxie.Schema.User<
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 1,
  etc...

Sorry this doesn’t answer the question, but IO.Inpsect() will always help you to narrow down the cause of the issue.

Hope this helps, and good luck.

Thank you all for your help in diagnosing the issue! I really appreciate the time and expertise you’ve shared.

As you suggested, the problem was that my create_story function wasn’t preloading the :user details. I’ve updated the code as recommended, and it now works perfectly.

Here’s the updated create_story function:

def create_story(attrs \\ %{}) do
  %Story{}
  |> Story.changeset(attrs)
  |> Repo.insert()
  |> case do
    {:ok, story} -> {:ok, Repo.preload(story, :user)}
    {:error, changes} -> {:error, changes}
  end
end

Thank you all again for your help!