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?