查询两个字段之和小于给定值的地方

要查找所有位置

sum + delta < 1000

pipe := c.Pipe(    []bson.M{        bson.M{"$project": bson.M{"_id": 1, "name": 1, "sum": 1, "delta": 1, "total": bson.M{"$add": []string{"$sum", "$delta"}}}},        bson.M{"$match": bson.M{"total": bson.M{"$lt": 1000}}},    })

这是工作代码：

package mainimport (    "fmt"    "gopkg.in/mgo.v2"    "gopkg.in/mgo.v2/bson")func main() {    session, err := mgo.Dial("localhost")    if err != nil {        panic(err)    }    defer session.Close()    session.SetMode(mgo.Monotonic, true) // Optional. Switch the session to a monotonic behavior.    c := session.DB("test").C("MarkModel")    c.DropCollection()    err = c.Insert(&MarkModel{bson.NewObjectId(), "n1", 10, 1}, &MarkModel{bson.NewObjectId(), "n2", 20, 2},        &MarkModel{bson.NewObjectId(), "n1", 100, 1}, &MarkModel{bson.NewObjectId(), "n2", 2000, 2})    if err != nil {        panic(err)    }    pipe := c.Pipe(        []bson.M{ bson.M{"$project": bson.M{"_id": 1, "name": 1, "sum": 1, "delta": 1,     "total": bson.M{"$add": []string{"$sum", "$delta"}}}}, bson.M{"$match": bson.M{"total": bson.M{"$lt": 1000}}},        })    r := []bson.M{}    err = pipe.All(&r)    if err != nil {        panic(err)    }    for _, v := range r {        fmt.Println(v["_id"], v["sum"], v["delta"], v["total"])    }    fmt.Println()}type MarkModel struct {    ID    bson.ObjectId `json: "_id,omitempty" bson: "_id,omitempty"`    Name  string        `json: "name" bson: "name"`    Sum   int`json: "sum" bson: "sum"`    Delta int`json: "delta" bson: "delta"`}

输出：

ObjectIdHex("57f62739c22b1060591c625f") 10 1 11ObjectIdHex("57f62739c22b1060591c6260") 20 2 22ObjectIdHex("57f62739c22b1060591c6261") 100 1 101