A simple example of how to use $lookup in Golang using mgo.

mongodb_lookup.go

package main
import (
        "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()

        // Optional. Switch the session to a monotonic behavior.
        session.SetMode(mgo.Monotonic, true)

        collection := session.DB("database").C("collection")
        pipeline := []bson.M{ 
                bson.M{"$match": bson.M{"_id": bson.ObjectIdHex("56b9df0c1e930a99cb2c33e9")}},
		            bson.M{"$lookup": bson.M{ "from": "localCollection", "localField": "localField", "foreignField": "foreignField", "as": "resultField"}},
       	}
        pipe := collection.Pipe(pipeline)
        resp := []bson.M{}
	      err = pipe.All(&resp)
       
       	if err != nil {
  		      fmt.Println("Errored: %#v \n", err)
	      }
	      fmt.Println(resp)
}