title: Rails subquery always returns nil value
body: <p>I need to count rows from other table in subselect, so I use this query:</p>
  
  <pre><code>follows_sql = Follow.where(followable_type: 'Idea').where('follows.followable_id = ideas.id').select('COUNT(followable_id)').to_sql
  
  idea = Idea.select("(#{follows_sql}) AS fcnt").includes(:collaborations).
    where(collaborations: { user_id: 4, owner: true })
  </code></pre>
  
  <p>So its produced valid SQL, but I cant access 'fcnt' value from idea var. I tried do it in different ways like:</p>
  
  <pre><code>idea[0].fcnt   # return nil
  idea[0]["fcnt"]  # return nil
  </code></pre>
  
  <p>But I only can access fields which exists in a Idea model.
  How I can access my custom '<strong>fcnt</strong>' field?
  <a href="https://i.stack.imgur.com/lPctY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lPctY.png" alt="enter image description here"></a></p>