title: How to query with '$in' over '_id' in reactive mongo and play
body: <p>I have a project set up with <code>playframework 2.2.0</code> and <code>play2-reactivemongo 0.10.0-SNAPSHOT</code>. I'd like to query for few documents by their ids, in a fashion similar to this:</p>
  
  <pre><code>def usersCollection = db.collection[JSONCollection]("users")
  val ids: List[String] = /* fetched from somewhere else */
  val query = ??
  val users = usersCollection.find(query).cursor[User].collect[List]()
  </code></pre>
  
  <p>As a query I tried:</p>
  
  <pre><code>Json.obj("_id" -&gt; Json.obj("$in" -&gt; ids))                        // 1
  Json.obj("_id.$oid" -&gt; Json.obj("$in" -&gt; ids))                   // 2
  Json.obj("_id" -&gt; Json.obj("$oid" -&gt; Json.obj("$in" -&gt; ids)))    // 3
  </code></pre>
  
  <p>for which first and second return empty lists and the third fails with error <code>assertion 10068 invalid operator: $oid</code>.</p>