Sorry I couldn’t come up with a better title for this one!

I’m trying to use an Ecto Changeset to maintain a set of changes provided by a user, eventually, when the user clicks “Save”, the changes will be written to the database but for now I’m trying to use the changes to keep track of all requested changes so I can conditionally show/hide form elements and display validation messages. Each time the user makes a change to a field, I don’t necessarily receive all the params for all the other fields again. So whenever I get a change from the user, I add that change to a changeset that is already tracking all previous changes. I’m getting some unexpected behaviour though. It occurs in the case that the user edits a field of an embedded association, changing it back to its original value.

  defmodule Page do
      use Ecto.Schema
      import Ecto.Changeset


      schema "pages" do
        embeds_one :background, Background, on_replace: :update do
          field :color, :string
         end
         timestamps()
      end

      def background_changeset(schema, params) do
        schema
        |> cast(params, [:color])
      end

      @doc false
      def changeset(schema, attrs) do
        schema
        |> cast(attrs, [])
        |> cast_embed(:background, with: &background_changeset/2)
      end
    end


    test "replacing a change in a changeset" do
      page = %Page{background: %Page.Background{color: "red"}}

      changed_to_blue = Page.changeset(page, %{"background" => %{"color" => "blue"}})
      assert changed_to_blue.changes.background.changes == %{color: "blue"}

      changed_to_green = Page.changeset(changed_to_blue, %{"background" => %{"color" => "green"}})
      assert changed_to_green.changes.background.changes == %{color: "green"}

      changed_back_to_red = Page.changeset(changed_to_green, %{"background" => %{"color" => "red"}})
      assert changed_back_to_red.changes.background.changes == %{color: "red"}

      # Assertion with == failed
      #  code:  assert changed_back_to_red.changes.background.changes == %{color: "red"}
      #  left:  %{color: "green"}
      #  right: %{color: "red"}
    end
  end

As you can see above, we can keep applying new changes to a changeset and they overwrite the previous changes, this is what I want. However, if we try to change the value back to the value contained in the Changeset’s data, the change is ignored.

Does this seem like unexpected behaviour? If so, I can post this as a Github issue for Ecto. Or am I misunderstanding/misusing changesets here?

Thanks!