[{"_id":1,"name":"John","contacts":[{"name":"henry","age":"22"},{"name":"merry","age":"12"}]},{"_id":2,"name":"John2","contacts":[{"name":"henry","age":"22"},{"name":"merry","age":"22"}]}]

db.collection.aggregate([{"$match":{"$expr":{"$in":[true,{"$map":{"input":"$contacts.age","in":{"$lte":[{"$toInt":"$$this"},20]}}}]}}}])