gistfile1.rb

# Controller
class GuestsController < ApplicationController

  def create
    @guest = Guest.new(params[:guest])
    if @guest.save
      redirect_to :action => 'index'
    else
      render :action => 'new'
    end
  end

end

# Model
class Guest
  validates_presence_of :student_id

  def student_name=(name)
    student = Student.find_by_name(name)
    self.student_id = student.id unless student.nil?
  end
end