Ecto: How to force overwrite embeds_many update

I’ve recreated a minimal example of the problem I’m experiencing, where cast_embed on a embeds_many is trying to protect me from accidentally overwriting my embeds when I don’t want it to. Given the following code:

defmodule TaskList do
  primary_key false
  embedded_schema do
    embeds_many :tasks, Task
  end

  def changeset(data, params \\ %{}) do
    data
    |> cast(params, [])
    |> cast_embed(:tasks)
  end
end

defmodule Task do
  @primary_key false
  embedded_schema do
    field: title
  end

  def changeset(data, params \\ %{}) do
    data
    |> cast(params, [:title])
    |> validate_required(:title)
  end
end

# Below called when LiveView form submitted

def handle_event("submit", params, socket) do
  # task_list already has 3 Task structs assigned to `tasks`, when loaded from DB
  task_list = socket.assigns.task_list

  result =
    task_list
    |> TaskList.changeset(params["data"])
    |> Repo.update()

  case result do
    {:ok, task_list} ->
      # redirect, etc
    {:error, changeset} ->
      # handle error
  end
end

TaskList.changeset/2 will raise an exception because I’m trying to replace the existing 3 Task embeds with new ones, and on_replace defaults to :raise.

Elsewhere in my app, I’m using embeds_one :foo, Foo, on_replace: :update , which has exactly the semantics I want, however embeds_many does not support this option, and none of the available options (:raise, mark_as_invalid, :delete) has the correct semantics. I would just like any cast_embed(:tasks) to “update/replace” what is there.

Right now I’m doing a dance where I replace the existing list with an empty list if I will be calling cast_embed, but it feels like a hack, and I’m replicating this hack in multiple places, and it makes the code quite messy and hard to reason about (esp when there’s other logic that conditionally calls cast_embed).

Any ideas how to do this simply?

This sounds like the embedded-schema version of the difference between cast_assoc and put_assoc; I’ve never used it, but would put_embed solve this?

If you don’t use primary keys what would be the difference between on_replace: :delete and on_replace: :update in the first place? This is besides the fact that for embeds_many you cannot use :update for replaced items, because there’s no way to know which old item shall be updated with which set of new data. Even less given you don’t have primary keys on your embeds.

You are correct, :delete has the semantics I was looking for, but something else in my code led me to believe it was “deleting” them in a way I wasn’t expecting. It wasn’t. Thanks for the push to look back at that approach!