Solved the first requirement as follows:

@spec check_primary(Ecto.Changeset.t()) :: Ecto.Changeset.t()
defp check_primary(%{valid?: true} = changeset) do
    registration = apply_changes(changeset)
    case Enum.count(registration.participants, fn participant -> participant.primary end) do
        1 -> changeset
        _ -> add_error(changeset, :base, "One primary registrant required")
    end
end

defp check_primary(changeset), do: changeset

not sure if this is the best approach though. @idi527 thanks for all your help.

Maybe something like Ecto.Multi might help. It would look like this

alias Ecto.Multi

@spec create_registration(%{binary => term} | %{atom => term}) :: {:ok, %{registration: %Registration{}, check_primary: :ok}} | {:error, :registration | :check_primary, Ecto.Changeset.t | :error, changes_so_far :: %{optional(atom) => term}}
def create_registration(attrs) do
  Multi.new()
  |> Multi.insert(:registration, Registration.changeset(%Registration{}, attrs))
  |> Multi.run(:check_primary, fn %{registration: registration} -> check_primary(registration) end)
  |> Repo.transaction()
end

which would rollback if check_primary(registration) returns :error.

Checking them separately does not help.

Why? I thought it’d work … Can you show the code that you’ve used?

I need to compute fees for the participant but that can happen only after arrival_date and departure_date changes, if any, are applied to the record

That also could be done either in a multi or through a changeset, I prefer changesets:

@spec put_fees(Ecto.Changeset.t) :: Ecto.Changeset.t
defp put_fees(%{valid?: true, changes:%{arrival_date: arrival_date, departure_date: departure_date}} = changeset) do
  put_change(changeset, :fees, compute_fees(arrival_date, departure_date))
end
defp put_fees(changeset), do: changeset