[{"type":"example","level1":[{"name":"Bill","level2":[{"skill":"gardener"},{"skill":"carpenter"}]},{"name":"John","level2":[{"skill":"painter"},{"skill":"acrobatics"}]}]}]

db.collection.aggregate([{"$unwind":"$level1"},{$match:{"level1.level2.skill":"acrobatics"}},{"$group":{_id:"$_id",level1:{"$addToSet":"$level1"}}}])