title: spring-data specifications - return list of ids instead of objects
body: <p>i'm looking to get all ids of all subscribed users. this is my predicate:</p>
  
  <pre><code>public static Specification&lt;User&gt; userSubscribed(){
    return new Specification&lt;User&gt;() {
      public Predicate toPredicate(Root&lt;User&gt; root, CriteriaQuery&lt;?&gt; query, CriteriaBuilder builder) {
        return builder.equal(root.get("status"), 1 /* 1 = subscribed */);
      }
    };
  }
  </code></pre>
  
  <p>My repository, however, only allows count, findAll and findOne. Is there a way to get it to return a list of a certain field?</p>