create.ex

def create(_parent, arguments, _resolution) do
  %{name: name, subject_id: subject_id} = arguments

  tag = Repo.insert(Tag.changeset(%Tag{}, %{name: name}))
    |> case do
      {:ok, tag} -> tag
      {:error, _} -> Repo.one(from(tag in Tag, where: tag.name == ^name))
    end

  subject_model = case arguments do
    %{subject_type: :establishment} -> Poutineer.Models.Establishment
    %{subject_type: :review} -> Poutineer.Models.Review
    %{subject_type: :recipe} -> Poutineer.Models.Recipe
    %{subject_type: :menu_item} -> Poutineer.Models.MenuItem
  end

  subject = Repo.get!(subject_model, subject_id) |> Repo.preload(:tags)

  if tag && subject do
    subject
      |> Ecto.Changeset.change()
      |> Ecto.Changeset.put_assoc(:tags, [tag | subject.tags])
      |> Repo.update()
      |> case do
        {:ok, _} -> {:ok, tag}
        {:error, _} = error -> error
      end
  end
end