mongo 保留两位小数

{ "_id" : 1, "a" : 1, "b" : 2, "c" : 3 }
{ "_id" : 2, "a" : 10, "b" : 12, "c" : 13 }
db.test.aggregate({"$project":{"d":{"$divide":[{"$trunc":{"$multiply":[{"$divide":["$b","$c"]},100]}},100]}}})
{ "_id" : 1, "d" : 0.66 }
{ "_id" : 2, "d" : 0.92 }