title: Rails 3: create_association fails validation and deletes previous associated record
body: <p>Let's assume I have two models.</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_one :blog
  end
  
  class Blog &lt; ActiveRecord::Base
    belongs_to :user
  
    validates_presence_of :user
    validates_uniqueness_of :user_id
  end
  </code></pre>
  
  <p>Let's assume I have one user with a blog. For some reason, let's pretend I call <code>create_blog</code> for the same user (I know it should not be an option since the user already has a blog). If this blog doesn't pass the validations, and it won't, not only isn't saved, but it deletes the previous blog the user has.</p>
  
  <p>Why is this happening? Why the initial blog gets deleted? Is this behavior expected or is there something I missing?</p>