title: Ruby on Rails ActiveRecord "has_many :through" uniqueness validation
body: <p>Currently I insert a new relationship by everytime checking, if it doesn't exist:</p>
  
  <pre><code>unless Relationship.exists?(:entry_id =&gt; entry.id, :tag_id =&gt; tag.id)
  </code></pre>
  
  <p>How could I implement such validation inside the Relationship model, so that it wouldn't allow to have more than one relationship between the same entry and tag?</p>