I need to get a single column values which are distinct. My table has three fields & none of them is primary.

id, segment, active. I had written the following code, but it returns a result of size '0'.

DBObject target = new BasicDBObject();
target.put("segment", segmentId);
target.put("active", "true");

List<String> result = targetList.distinct("id",target)

The value of result.size() is 0