Database
[{"collection":"collection","count":10,"content":[{"k":{"type":"int","minInt":0,"maxInt":10},"a":{"type":"int","minInt":0,"maxInt":10},"b":{"type":"int","minInt":0,"maxInt":10},},{}]}]
Query
db.collection.aggregate([{"$unwind":{"path":"$content","preserveNullAndEmptyArrays":true}},{"$project":{"_id":0,"content":1}}])