MongoDb:Como inserir objeto adicional na coleção de objetos em golang?

session, err := mgo.Dial("127.0.0.1")
if err != nil {
    panic(err)
}

defer session.Close()

session.SetMode(mgo.Monotonic, true)

// Drop Database
if IsDrop {
    err = session.DB("test").DropDatabase()
    if err != nil {
        panic(err)
    }
}

// Collection Stack
c := session.DB("test").C("stack")

// Query
query := bson.M{"ownerEmail": "[email protected]"}
update := bson.M{"$push": bson.M{"camps": bson.M{"name": "cubs-killeen", "location": "some other Place"}}}

// Update
err = c.Update(query, update)
if err != nil {
    panic(err)
}