title: Cannot encode object in pymongo
body: <p>I have this query in mongodb:</p>
  
  <pre><code>db.getCollection('users').find(
                              {"first_name": {$in: ['Alex', 'Andrew']}},
                              {'id': 1, '_id': 0}
                              )
  </code></pre>
  
  <p>And have results from this query.
  But when I try to run this query in python with pymongo:</p>
  
  <pre><code>select_users_id = collection_users.find(
      {"first_name": {"$in: ['Alex', 'Andrew']"}},
      {"id": 1, "_id": 0}
  )
  
  for i in select_users_id:
      print i.values()
  </code></pre>
  
  <p>I receive error message:</p>
  
  <pre><code>bson.errors.InvalidDocument: Cannot encode object: set(["$in: ['Alex', 'Andrew']"])
  </code></pre>