MongoDB Java Driver - Use exists projection in find query

Question

I want to get all documents where the field download does not exists

find{ "download" : {$exists: false}}

For Java I found an Example:

  BasicDBObject neQuery = new BasicDBObject();
  neQuery.put("number", new BasicDBObject("$ne", 4));
  DBCursor cursor = collection.find(neQuery);

  while(cursor.hasNext()) {
    System.out.println(cursor.next());
  }

My Adaption is

      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())

The Exists Line is not working:

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

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.

Answer 1

you use

collection.find(query,field);

the DBObject that is the 2nd parameter to the find method is used to indicate which attributes of the resulting documents you want to be returned. It is useful for reducing network load.

In your example, you try to put an $exists clause into the field DBObject. That won't work. The 2nd parameter object can only have attribute values of 1(include this attribute) or 0(exclude).

Put it into the first DBObject called query instead.

Also see here and here