new_student.html.erb

<%= f.text_field :pending_room_number, :value => @student.room_number %>
<%= f.collection_select :residence_id, Residence.all, :id, :name %>

room.rb

class Room < ActiveRecord::Base
  belongs_to :residence
  has_many :students
end

student.rb

class Student < ActiveRecord::Base
  belongs_to :room
  delegate :number, :residence, :to => :room, :prefix => true, :allow_nil => true 
  attr_accessor :pending_room_number, :residence_id

  before_save :assign_room

  def assign_room
    self.room = if room = Room.find_by_number_and_residence_id(pending_room_number, residence_id)
                  room
                else
                  Room.create! { :number => pending_room_number, :residence_id => residence_id}
                end unless self.room.present?
  end
end