form_controller

# CONTROLLER
def create(conn, %{"expense" => expense_params}) do
    user = conn.assigns.current_user
    case Finapp.Records.expense_create(expense_params, user) do
      {:ok, _expense} ->
        redirect(conn, to: Routes.expense_path(conn, :index))
      {:error, changeset} ->
        tags = Finapp.Records.tag_list(user)
        render(conn, "new.html", changeset: changeset, tags: tags)
    end
    redirect(conn, to: Routes.expense_path(conn, :new))
  end

# FORM
<div class="field">
        <%= label f, :tag_id, "Categorias" %>
        <%= multiple_select f, :tag_id, Enum.map(@tags, &{&1.name, &1.id}), class: "ui fluid search dropdown tag-menu" %>
        <%= error_tag f, :tag_id %>
</div>