I am using Morphia Java driver for querying a MongoDB that contains a collection of the following form:
MyCollection {
TypeA
TypeB
}
I want to retrieve all distinct values of TypeB which I do using the following code:
DBCollection myCol = getDatastore().getCollection(MyCollection.class);
List typeBs = myCol.distinct("TypeB");
Above code works as expected, but list of distinct values is of course not sorted.
I have experimented with the following code:
DBCollection myCol = getDatastore().getCollection(MyCollection.class);
DBObject orderBy = new BasicDBObject("$orderby", new BasicDBObject("TypeB", 1);
List typeBs = myCol.distinct("TypeB", orderBy);
But in this case the list is empty, where are my assumptions wrong?
UPDATE
By using the CLI I found out that the following query returned the expected result:
> db.mycollection.find({$query : {}, $orderby : { TypeB : 1 }})
So I adjusted my code accordingly:
DBCollection myCol = getDatastore().getCollection(MyCollection.class);
BasicDBObject ascending = new BasicDBObject("TypeB", 1);
BasicDBObject filter = new BasicDBObject();
filter.put("$query", new BasicDBObject());
filter.put("$orderby", ascending);
List typeBs = myCol.distinct("TypeB", filter);
Still result contains 0 entries!
What really makes me confused is that the same query works if I use .find instead of .distinct:
DBCollection myCol = getDatastore().getCollection(MyCollection.class);
BasicDBObject ascending = new BasicDBObject("TypeB", 1);
BasicDBObject filter = new BasicDBObject();
filter.put("$query", new BasicDBObject());
filter.put("$orderby", ascending);
myCol.find(filter).hasNext(); <-- Evaluates to TRUE
Why is it that the filter doesn't work when using the distinct method-call?
the second DBObject parameter of DBCollection.distinct() is a query object, used to define the matching criteria. To sort the list distincts you can use:
List typeBs = myCol.distinct("TypeB");
java.util.Collections.sort(typeBs);