changeset.ex

def validate_version(cs = %{
                                      valid?: true, 
                                      changes: %{definition: new_def, version: client_version}, 
                                      data: %{definition: old_def, version: db_version}}) 
                                      when client_version > db_version do
    case Map.equals(new_def, old_def) do
      true ->
        put_change(cs, :version, version+1)
      false ->
        add_error(cs, :definition, "definition_out_of_sync")
    end
  end

  def validate_version(c = %{data: %{version: version}}), do: put_change(c, :version, version+1)