title: MongoDB Java Driver - Use exists projection in find query
body: <p>I want to get all documents where the field download does not exists</p>
  
  <pre><code>find{ "download" : {$exists: false}}
  </code></pre>
  
  <p>For Java I found an Example:</p>
  
  <pre><code>  BasicDBObject neQuery = new BasicDBObject();
    neQuery.put("number", new BasicDBObject("$ne", 4));
    DBCursor cursor = collection.find(neQuery);
  
    while(cursor.hasNext()) {
      System.out.println(cursor.next());
    }
  </code></pre>
  
  <p>My Adaption is</p>
  
  <pre><code>      BasicDBObject field = new BasicDBObject();
        field.put("entities.media", 1);
    field.put("download", new BasicDBObject("$exists",false));
    System.out.println("Start Find");
    DBCursor  cursor = collection.find(query,field);      
        System.out.println("End Find Start Loop ALL 100k");
     int i = 1;
                  while(cursor.hasNext())
  </code></pre>
  
  <p>The Exists Line is not working:</p>
  
  <pre><code>Exception in thread "main" java.lang.reflect.InvocationTargetException
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
          at java.lang.reflect.Method.invoke(Method.java:616)
          at org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader.main(JarRsrcLoader.java:58)
  Caused by: com.mongodb.MongoException: Unsupported projection option: $exists
          at com.mongodb.MongoException.parse(MongoException.java:82)
          at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:314)
          at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:295)
          at com.mongodb.DBCursor._check(DBCursor.java:368)
          at com.mongodb.DBCursor._hasNext(DBCursor.java:459)
          at com.mongodb.DBCursor.hasNext(DBCursor.java:484)
          at ImgToDisk.main(ImgToDisk.java:61)
          ... 5 more
  </code></pre>
  
  <p>It have no clue right now which the right adaption would be, since I got my query working in the shell and with UMongo, the transfer to java seems to be not so easy to see.</p>