title: Why would association.build not assign parent_id to it child?
body: <p>I've got the following models:</p>
  
  <p>In plan.rb</p>
  
  <pre><code>has_many :tickets
  </code></pre>
  
  <p>and in ticket.rb</p>
  
  <pre><code>belongs_to :plan
  validates_presence_of :plan_id
  </code></pre>
  
  <p>When executing the following code in the controller </p>
  
  <pre><code>@plan.tickets.build( ... )
  @plan.save
  </code></pre>
  
  <p>save fails with error_message for ticket: plan can't be blank. (plan is valid.)</p>