Dynamically add and delete nested model data in a LiveView form

Phoenix Forum Questions / Help
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Ecto noob question here. I followed this tutorial for dynamically adding and deleting the many side of a one-to-many relationship in a Phoenix LiveView: Nested model forms with Phoenix LiveView - Tutorials and screencasts for Elixir, Phoenix and LiveView

Just to keep it easy, lets say I have a one-to-many relationship between a Blog and its Comments.

Using the example in the above tutorial, I can dynamically add multiple Comments to a Blog. I can also delete Comments. If I delete a Comment that is in the changset, but not committed to the database yet (one with a temp ID), then I can continue to add more Comments. However, if I delete a Comment that is already in the database and then try to add another Comment, I get this error:

(RuntimeError) cannot replace related %Test.Blogs.Entry{__meta__: #Ecto.Schema.Metadata<:loaded, "comments">, delete: nil, id: 1, inserted_at: ~N[1970-01-01 00:00:00], blog: #Ecto.Association.NotLoaded<association :blog is not loaded>, blog_id: 1, temp_id: nil, title: "Test Blog", updated_at: ~N[1970-01-01 00:00:00]}. This typically happens when you are calling put_assoc/put_embed with the results of a previous put_assoc/put_embed/cast_assoc/cast_embed operation, which is not supported. You must call such operations only once per embed/assoc, in order for Ecto to track changes effeciently [sic].

My code is exactly the same as the code in the tutorial at this point, only with different table names. I’m using the latest versions of Phoenix, LiveView, and Ecto.

I’ve scoured the Ecto documentation for about 10 hours now, but I haven’t found a way to get past this error using Ecto changesets, as described in the tutorial. Is there a way to fix this error, or is it just impossible to use Ecto in the way this tutorial suggests?

1 Like
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I misspoke in my original post. The tutorial I mentioned doesn’t show how to delete existing “Comments” from a “Blog.” I added that using this function:

def handle_event("remove-existing-comment", %{"remove" => id}, socket) do
    deleted_comments = [ id | socket.assigns.deleted_comments ]
    Blog.delete_comment(id)

    comments =
      socket.assigns.blog.commets
      |> Enum.reject(fn comment ->
        comment.id in deleted_comments
      end)

    changeset =
      socket.assigns.changeset
      |> Ecto.Changeset.put_assoc(:comments, comments)

    {:noreply, assign(socket, changeset: changeset, deleted_comments: deleted_comments)}
  end

This works fine, but if I delete an existing comment using this callback, then the function to create new Comments stops working, with the error I mentioned in my original post.

I apologize for attributing this part of the code to the author of the tutorial.

So, again, is there a way to get this to work with Ecto, or is it an impossible scenario?