Get_field returning nil instead of my default

My schema has two numbers, they must add up to 100 on save.

The fields are simple field ... :integer with no defaults, etc.

I have a custom validator, to get the fields and sum them:

  defp validate_sum(changeset) do
    Enum.reduce([:part_a,  :part_b], 0,
      fn key, sum ->
        sum + (get_field(changeset, key, 0)) # drama generator
      end
    )
    |> case do
      100 ->
        changeset
      _ ->
        changeset
        |> add_error(:part_a, "sum of parts must be 100")
        |> add_error(:part_b, "sum of parts must be 100")
    end
  end

My problem is that get_field returns nil, not zero. I think because the key exists, but its value is nil?

I can solve my problem by writing (get_field(changeset, key) || 0), which returns the value if its not nil, or 0.

Is this the correct way of handling this? Is get_field intended to only return it’s “default” value if you try to access a non existent / misnamed field? Why would that be useful?