title: preload nested association query
body: <p>I'm trying to order by in a query preload. This is my query:</p>
  
  <pre><code>query =
        from(
          p in Project,
          preload: [rows: {from(r in Row, order_by: r.index), [images: from(r in Row, order_by: r.index)}]],
          where: p.user_id == ^user_id
        )
  </code></pre>
  
  <p>But this doesn't work and gives me a syntax error:</p>
  
  <pre><code>** (SyntaxError) web/controllers/project_controller.ex:11: "{" is missing terminator "}". unexpected token: ")" at line 13
  </code></pre>
  
  <p>Does anybody know how to preload nested attributes with Ecto?</p>