custom_assert_04.ex

@doc """
Asserts that the given changeset is invalid, and that when the
assertion_expression is applied to the error_message it results in a truthy
value.
"""
defmacro assert_invalid(changeset, field, assertion_expression) when is_atom(field) do
  expr = Macro.to_string(assertion_expression)

  quote do
    error_messages = unquote(changeset)
    |> Ecto.Changeset.traverse_errors(fn {message, _opts} -> message end)
    |> Map.get_lazy(unquote(field), fn ->
        flunk("field :#{unquote(field)} not found in errors")
    end)

    error_messages
    |> Enum.any?(fn message ->
      var!(error_message) = message
      unquote(assertion_expression)
    end)
    |> unless do
      flunk("""
      Expression did not match error message

      #{IO.ANSI.cyan()}error_message:#{IO.ANSI.reset()} #{inspect(error_messages)}
      #{IO.ANSI.cyan()}expression:#{IO.ANSI.reset()} #{unquote(expr)}
      """)
    end
  end
end