Ecto embed timestamps inserted_at updated

So I have this embed structure:

defmodule MyApp.Struct do
  use Ecto.Schema
  import Ecto.Changeset
  
  @primary_key false
  @derive Jason.Encoder
  embedded_schema do
    field(:name, :string)
    field(:completed_at, :utc_datetime)
    timestamps()
  end

end

I wanted to have inserted_at and updated_at fields set, so I added the timestamps() line.

It seems like the updated_at value works correctly, but the inserted_at keeps on being updated on each update, which is not what I want.

Here’s how I do the update, from the Holder struct:

defmodule MyApp.Holder do
  schema "holder" do
    embeds_one(:struct, MyApp.Struct, on_replace: :delete)
  end

  def save_struct(holder, struct_params) do
    holder
    |> change()
    |> put_embed(
      :struct,
      MyApp.Holder.changeset(
        holder.struct || %MyApp.Holder{},
        struct_params
      )
    )
    |> Repo.update()
  end

end

I thought this would have to do with put_embed vs cast_embed, but with cast_embed, I need to pass all struct_params, which is not what I want.

So I went for a manual solution:

    field(:inserted_at, :utc_datetime)
    field(:updated_at, :utc_datetime)

  def changeset(struct, params) do
    struct
    |> cast(params, [:name, :completed_at])
    |> cast(%{inserted_at: struct.inserted_at || Timex.now(), updated_at: Timex.now()}, [:inserted_at, :updated_at])
  end

But this is much less elegant.

What am I doing wrong? Or is this because of the embed thing?

Thanks