function upsert_user_projects.ex

  def upsert_user_projects(user_id, project_ids) when is_list(project_ids) do
    {:ok, time} = Ecto.Type.cast(:naive_datetime, Timex.now())

    project_users =
      project_ids
      |> Enum.uniq()
      |> Enum.map(fn project_id ->
        %{
          user_id: user_id,
          project_id: project_id,
          inserted_at: time,
          updated_at: time
        }
      end)

    multi =
      Multi.new()
      |> Multi.delete_all(
        :user_project_deleted,
        UserProject
        |> where([user_project], user_project.user_id == ^user_id)
      )
      |> Multi.insert_all(:user_project_inserted, UserProject, project_users)

    case Repo.transaction(multi) do
      {:ok, _multi_result} ->
        {:ok, Accounts.get_user(user_id)}

      {:error, changeset} ->
        {:error, changeset}

      {:error, _, changeset, _} ->
        {:error, changeset}
    end
  end