title: Best practice about empty belongs_to association
body: <p>Imagine the following situation:</p>
  
  <p>I have a <code>dog</code> model and a <code>house</code> model. A dog can belong to a house, and a house can have many dogs, so:</p>
  
  <pre><code>Class Dog &lt;  ActiveRecord::Base
    belongs_to :house
  end
  
  Class House &lt; ActiveRecord::Base
    has_many :dogs
  end
  </code></pre>
  
  <p>Now, imagine that I also want to create dogs that don't have a house. They don't belong to house. Can I still use that relationship structure and simply don't inform a <code>:house_id</code> when creating it?</p>
  
  <p>Is there a better practice?</p>
  
  <p>Obs.: I used this analogy to simplify my problem, but my real situation is: I have a model a user can generate instances of it. He can also create collections of those instances, but he can leave an instance outside a collection.</p>