title: Rails Active Record - :includes with only specific fields
body: <p>I'm working on a simple API.  I'd like this index method to return only two or three fields from the person table, rather than every field from  that table.</p>
  
  <pre><code>api :GET, "/v2/branches/", "Return every branch."
    def index
      @branches = Branch.includes(:people, :social_link).where("branches.company_id = ?", @company.id)
      render json: @branches.to_json(include: [:people, :social_link])
    end
  </code></pre>
  
  <p>Rather than returning each persons first born child, i'd just like it to show first_name, last_name, email_address...  Any ideas?</p>