Phoenix framework - Custom changeset validations

Question

I\'m really new to phoenix and elixir, so my apologies if these seem like simple questions. I\'ve searched stack overflow and blogs before I thought about posting it here.

Answer 1

I had to do this exact thing and it took me a bit of time to figure it out. I ended writing a custom validator for the changeset.

def changeset(model, params \\ :empty) do
  model
  |> cast(params, @required_fields, @optional_fields)
  |> validate_a_less_eq_b
end

defp validate_a_less_eq_b(changeset) do
  a = get_field(changeset, :a)
  b = get_field(changeset, :b)

  validate_a_less_eq_b(changeset, a, b)
end
defp validate_a_less_eq_b(changeset, a, b) when a > b do
  add_error(changeset, :max, "'A' cannot be more than 'B'")
end
defp validate_a_less_eq_b(changeset, _, _), do: changeset

You would, of course, want to use regular validators to ensure that a and b are valid numbers.