#blog_snippets

week_may4_validation03.ex

defp mathematical_validation(changeset) do
  revenue = get_field(changeset, :revenue)
  expense = get_field(changeset, :expense)
  net_gain = get_field(changeset, :net_gain)

  with {:is_valid, true} <- {:is_valid, changeset.valid?},
       {math_validation, true} <- 
         {math_validation, Decimal.sub(revenue, expense) == Decimal.new(net_gain)} do
    changeset
  else
    {:is_valid, false} -> changeset
    {math_validation, false} -> add_error(changeset, :net_gain, "must equal revenue minus expense")
  end
end