More precisely, I want the same functionality as search_as_you_type (example), except with word n-gram results, instead of retrieving the entire _source. For example, if the _source in the search_as_you_type field is "quick brown fox jump lazy dog", and my query is "brown f", then the result should be "brown fox", or "quick brown fox", not "quick brown fox jump lazy dog".
Also, I'd like to achieve this without additional indexing ie explicitly add suggest entries as with the completion suggester.