[{"_id":ObjectId("5e42e5337f8bc30a75985c81"),"a":1,"b":[{"a":1,"b":2},{"c":3,"d":4},{"e":5,"f":6}],"c":2},{"_id":ObjectId("5e42e5387f8bc30a75985d18"),"a":1,"b":[{"a":1,"b":2},{"c":3,"d":4},{"e":5,"f":6}],"c":2}]

db.collection.aggregate([{$addFields:{b:{$map:{input:"$b",in:{foo:"$$this"}}}}}])