This feature looks to be very popular feature request. https://github.com/elasticsearch/elasticsearch/issues/256
Unfortunately, awesome ES team has other priority to work as I noted from the above thread.
"Don't hold your breath waiting on Field Collapsing. It won't be part of the 0.90 final. The ES team has to do a substantial refactoring before they will be able to implement this feature in a satisfying way. Probably the best course of action is to implement your own, client-side, or investigate if you can accomplish the same goals using a Parent-Child design."
http://www.elasticsearch.org/guide/reference/query-dsl/top-children-query/