phoenix: enforce field in changeset

user.ex

def changeset(model, params \\ %{}) do
  model
  |> cast(params, [:name, :email] ++ coherence_fields())
  |> enforce_api_token
  |> validate_required([:name, :email])
  |> validate_format(:email, ~r/@/)
  |> unique_constraint(:email)
  |> unique_constraint(:api_token)
  |> validate_coherence_assent(params)
end

defp enforce_api_token(changeset) do
  if changeset.data.api_token == nil do
    changeset |> put_change(:api_token, (:crypto.strong_rand_bytes(32) |> Base.encode64 |> binary_part(0, 32)))
  else
    changeset
  end
end