title: Eager loading doesnt include has_one associations
body: <p>I have 3 models:</p>
  
  <pre><code>class Opportunity
     include Mongoid::Document
     ...
     ...
  
     has_many :proposals
  end
  
  class Proposal
     include Mongoid::Document
     ...
     ...
  
     belongs_to :opportunity
     has_one :risk
  
     scope :eager, -&gt;{ includes([:opportunity, :risk]) }
  
     def as_json(options={})
      attrs = super(:include =&gt; [:opportunity, :risk])
      attrs
    end
  end
  
  class Risk
     include Mongoid::Document
     ...
     ...
  
     belongs_to :proposal
  end
  </code></pre>
  
  <p>If I now do try eager load and render the result:</p>
  
  <pre><code>render json: {result: Proposal.eager.first()}
  </code></pre>
  
  <p>I get the proposal record including the opportunity but the risk record is missing. Am I doing something wrong??</p>
  
  <p>Also doing Proposal.includes(:risk, :opportunity).first() doesnt include the record from Risk. I checked the database - the record is there. And when I look at the risk_id field it has the right id there. I thought I can do eager loading with has_one relationships?</p>
  
  <p>Any idea? </p>