Getting the elements from Collection MongoDB using Golang and mgo

Question

My task is writing chat with history. So for creating history i need each of messages send to Mongodb and when I have a next connecting I need to getting all messages and, with loop, send to all clients that are connected to chat

This is code of my ChatServer

func ChatServer(ws *websocket.Conn) {

// Connecting to MongoDB, collection History
session, err := mgo.Dial("mongodb://******:*******@ds045795.mongolab.com:45795/catalog")
if err != nil {
    panic(err)
}
defer session.Close()
session.SetMode(mgo.Monotonic, true)
c := session.DB("catalog").C("History")

// fmt.Println(c.Find())
// Adding clients to the map
clientId := ws.RemoteAddr().String()
defer ws.Close()
clients[ws] = true

// Loop for receiving msg
for {
    var msg string
    // If can not read msg - delete client from map
    if err := websocket.Message.Receive(ws, &msg); err != nil {
        delete(clients, ws)
        return
    }
    sendAll(msg)
    err = c.Insert(&Connect{clientId, msg})
    if err != nil {
        log.Fatal(err)
    }
}
}

So my problem is getting all the elements in order from the collection. I have no idea how to do it, because can not find the proper function in documentations. Maybe you have some other offers?

Answer 1

First of all I agree with commenters above - you should add timestamp to your Connect structure. But even without it you can sort your entries by ObjectID as part of it is timestamp. Yes this is quite dirty way and may cause issues if you use sharding or transfer database to another server but in your case (single Mongolab instance) it's possible with something like like (I don't know your structure so "ip" and "message" are just assumptions)

var connects []Connect
c.Find(bson.M{"ip": "127.0.0.1"}).Sort("-_id").Limit(50).All(&connects) // 50 entries in desc order

for _, connect := range connects {
    log.Println(connect.Message)
}

But really add time to your Connect like that

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
    "log"
    "time"
)

type Connect struct {
    Ip      string
    Message string
    Ts      time.Time
}

func main() {
    session, err := mgo.Dial("mongodb://souser:[email protected]:55855/catalog")

    if err != nil {
        panic(err)
    }
    defer session.Close()
    session.SetMode(mgo.Monotonic, true)

    c := session.DB("catalog").C("History")

    for i := 0; i < 100; i++ {
        c.Insert(&Connect{"127.0.0.2", fmt.Sprintf("Test message #%d", i), time.Now()})
        if err != nil {
            log.Fatal(err)
        }
    }

    var connects []Connect
    c.Find(bson.M{"ip": "127.0.0.2"}).Sort("-ts").Limit(50).All(&connects)

    for _, connect := range connects {
        log.Println(connect.Message)
    }
}