title: has_one unitialized constant when using .build_
body: <p>I have situation where an assignment has one training session associated with it</p>
  
  <pre><code>class Assignment &lt; ActiveRecord::Base
     has_one :trainingsession
  end
  
  class TrainingSession &lt; ActiveRecord::Base
    belongs_to :assignment
    has_many :drills
  end
  </code></pre>
  
  <p>I keep getting a <code>uninitialized constant</code> error when I'm trying to build an object with a has_one relationship</p>
  
  <p>I'm using the following to build the training session in my controller</p>
  
  <pre><code>@activetrainingsession = @assignment.build_trainingsession 
  </code></pre>
  
  <p>And that line blows up with the <code>uninitialized constant</code></p>
  
  <p>Something that seems like it should be straightforward!!</p>