I’ll try to describe it as simple as possible.

I made a very simple project to play around with while trying to learn Elixir and Phoenlix Liveview. I created Categories and Products with phx.gen.live. A Product has a belongs_to-relationship with Category, and Category a has_many-relationship with Products.

Adding products and assigning category to them works fine. When hitting the save button the index lists all the products:

In my index-template I have the following code to list it as product name (category name).

<%= product.name %> (<%=product.category.name %>)

Hoever when I click edit on an item and change the category and hit save, the process crashes with the following error:

** (KeyError) key :name not found in: #Ecto.Association.NotLoaded<association :category is not loaded>

After the crashing process restarts everything works fine again, showing the correct new category in the index-view. I also get a quick red flash saying “Something went wrong”.

In live/product_live/index.ex I have modified the mount-function:

def mount(_params, _session, socket) do
  {:ok, stream(socket, :products, Products.list_products_test())}
end

And list_produts_test() is defined as:

def list_products_test do
  Repo.all(Product) |> Repo.preload([:category])
end

I guess I get the error because I have changed the category for one product, and when hitting save somehow the preload() is not called until the process crashes and is reloaded. Should I put the preload() somewhere else, will that solve my problem?