how best to make associations in controller?

new.html.haml

= simple_form_for(@trip) do |f|
  = f.input :from_place, :collection => @trip.owner.places
  = f.input :to_place, :collection => @trip.owner.places
  -# ...etc...

trip.rb

class Trip < ActiveRecord::Base
  belongs_to :from_place, foreign_key: 'from_place_id', class_name: Place
  belongs_to :to_place, foreign_key: 'to_place_id', class_name: Place
end

trips_controller.rb

class TripsController < ApplicationController
  def create
    # what's the best way to make the association here?
    # The form sends up IDs, so can either
    # (1) assign IDs directly w/ mass assignment, or
    # (2) look up the associated objects and add them to the hash (e.g. params[:trip][:from_place] = Place.find(x))
    @trip = Trip.new(params[:trip])

    respond_to do |format|
      # ...etc...
    end
  end

end