Error when listing posts with relations after updating

zingo

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?

6 comments

/phoenix #liveview #live-view #relationships

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2024-06-20 21:03:22 UTC

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al2o3cr

Can you show the code that handles the “save” event?

Post #1

zingo

I have not modified the generated code, which looks like this:

  def handle_info({TestingWeb.ProductLive.FormComponent, {:saved, product}}, socket) do
    {:noreply, stream_insert(socket, :products, product)}
  end

Edit: Or maybe it’s this function:

  def update_product(%Product{} = product, attrs) do
    product
    |> Product.changeset(attrs)
    |> Repo.update()
  end

Post #2

al2o3cr

I suspect the best place to change is the code between these two spots - the part that calls update_product and then sends the {:saved, product} message.

You’d want to add a Repo.preload there to ensure that the Product being added to the stream has the needed association loaded.

Post #3

zingo

I have now modified my code, it works, but my solution seems a bit sketchy.

First i created a modified version of the generated get_product():

The original:

def get_product!(id), do: Repo.get!(Product, id)

My modified version:

def get_product_2!(id), do: Repo.get!(Product, id) |> Repo.preload([:category])

Then I modified handle_info():

  def handle_info({TestingWeb.ProductLive.FormComponent, {:saved, product}}, socket) do
    product = Products.get_product_2!(product.id) # added this line
    {:noreply, stream_insert(socket, :products, product)}
  end

Would this be the proper way of doing it, or is there a neater way?

Post #4

al2o3cr

handle_info already has a Product struct, why Repo.get it again?

Post #5

zingo

I don’t know how to preload inside handle_info. I tried product = product |> Repo.preload([:category]) which didn’t work. Maybe if I alias Repo it will work. I’ll try that.

Edit: Yes it did work. Still learning, thanks :).

Post #6

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