Need to use $add and $gt in remove using mgo (Golang && MongoDB)

Question

I'm using Golang as my backend and MongoDB as the database for my web application. I'm in need of adding two values and check if the value in the database is greater than the added value and if it is, I need to remove the row from the MongoDB.
The code which I wrote is as follows

err, err1 := c.RemoveAll(bson.M{
        "currenttime": bson.M{
            "$gt": bson.M{
                "$add": []interface{}{time.Now().Unix(),"100"},
            },
        },
     })

FYI: I wrote the sample code for MongoDB but this is not working as well

db.Collection.remove(
   {currenttime:{
        $gt:{
                $add:[100,100]
             }
              }
    }
 )

Please let me know if this is possible. And please give me any alternative way to do this.
Thanks

Answer 1

1- If you need to remove rows with field e.g. time > t + 10 from the MongoDB, you do't need $add, you may use:

info, err := c.RemoveAll(bson.M{"time": bson.M{"$gt": t + 10}})
if err != nil {
    panic(err)
}

Here is the working code:

package main

import (
    "fmt"
    "time"

    "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("Model")
    c.DropCollection()
    t := time.Now().UTC().Unix()
    err = c.Insert(
        &Model{bson.NewObjectId(), "n1", t},
        &Model{bson.NewObjectId(), "n2", t + 50},
        &Model{bson.NewObjectId(), "n3", t + 100},
        &Model{bson.NewObjectId(), "n4", t + 150})
    if err != nil {
        panic(err)
    }
    r := []Model{}
    err = c.Find(nil).All(&r)
    if err != nil {
        panic(err)
    }
    fmt.Println(r)

    info, err := c.RemoveAll(bson.M{"time": bson.M{"$gt": t + 10}})
    if err != nil {
        panic(err)
    }
    fmt.Println(info)

    r2 := []Model{}
    err = c.Find(nil).All(&r2)
    if err != nil {
        panic(err)
    }
    fmt.Println(r2)
}

type Model struct {
    ID   bson.ObjectId `json: "_id,omitempty" bson: "_id,omitempty"`
    Name string        `json: "name" bson: "name"`
    Time int64         `json: "time" bson: "time"`
}

output:

[{ObjectIdHex("57f7354cc3176906c0ca7516") n1 1475818828} {ObjectIdHex("57f7354cc3176906c0ca7517") n2 1475818878} {ObjectIdHex("57f7354cc3176906c0ca7518") n3 1475818928} {ObjectIdHex("57f7354cc3176906c0ca7519") n4 1475818978}]
&{0 3 3 <nil>}
[{ObjectIdHex("57f7354cc3176906c0ca7516") n1 1475818828}]

---

2- For $add, see:
Query where sum of two fields is less than given value
https://docs.mongodb.com/manual/reference/operator/aggregation/add/