Idiomatic go way to compare 2 slices to see which element has been removed

I am using RethinkDB in my application and I have a lobby that has users.

RethinkDB has the ability to watch for changes to a table and when changes happen it automagically emits the changes so you can do whatever you want with the data, right now I'm trying to make it so when a user leaves the lobby I can send a websocket out to remove the user. The only thing is I'm trying to find out where the difference is in the before / after data which is a slice of members, this is the data:

type change struct {
	NewVal *fields `gorethink:"new_val,omitempty"`
	OldVal *fields `gorethink:"old_val,omitempty"`
}

type fields struct {
	ID      string `gorethink:"id"`
	Owner   string `gorethink:"owner"`
	Inqueue bool   `gorethink:"inqueue"`
	Members []struct {
		SteamID  string `gorethink:"steamid"`
		Username string `gorethink:"username"`
	} `gorethink:"members"`
	Messages []struct {
		Username  string    `gorethink:"username"`
		Message   string    `gorethink:"message"`
		CreatedAt time.Time `gorethink:"createdAt"`
	} `gorethink:"messages"`
}

Right now I'm doing

func (l *lobby) watchChanges() {
	db := common.DB()
	query := gorethink.Table("Lobbys").Get(l.ID).Changes()
	res, err := query.Run(db)
	if err != nil {
		log.Println(err)
	}

	go func(res *gorethink.Cursor, l *lobby) {
		defer res.Close()
		changes := new(change)
		for res.Next(&changes) {
			if changes.NewVal != nil && changes.OldVal != nil {
				switch {
				case len(changes.NewVal.Members) > len(changes.OldVal.Members):
					// Member has joined so announce who it was.
					
				case len(changes.NewVal.Members) < len(changes.OldVal.Members):
					// Member has left so announce who it was.
           -------->
				case len(changes.NewVal.Messages) > len(changes.OldVal.Messages):
					// New Message was recieved so announce the message.

				}
			}
		}
	}(res, l)

	select {
	case <-l.KillMe:
		res.Close()
		break
	}
}

The new entries are easy enough I'll just take the end off the slice and send that, but when it comes to the user leaving, how can I compare the changes.NewVal.Members and changes.OldVal.Members to see which index was removed so I can send the right member to remove through the websocket. Hope my question is clear let me know if it isn't.

This is how I'm currently doing it

removedIndex := 0
for i, oldMember := range changes.OldVal.Members {
    foundMissing := true
    for _, newMember := range changes.NewVal.Members {
        if reflect.DeepEqual(oldMember, newMember) {
            foundMissing = false
        }
    }
    if foundMissing {
        removedIndex = i
        break
    }
}

but it feels a bit hacky, is there a better way?

Sort the old and new members by a unique and sortable key. It looks like SteamID might be suitable for this purpose. Iterate through both slices checking for added and deleted elements by comparing the keys.

func diff(old []*member, new []*member) {
  sort.Sort(bySteamID(old))
  sort.Sort(bySteamID(new))
  i, j := 0, 0
  for i < len(old) && j < len(new) {
	switch {
	case old[i].SteamID < new[j].SteamID:
		fmt.Println(" delete", old[i].SteamID)
		i++
	case old[i].SteamID > new[j].SteamID:
		fmt.Println(" add", new[j].SteamID)
		j++
	default:
		i++
		j++
	}
  }
  for i < len(old) {
	fmt.Println(" delete", old[i].SteamID)
	i++
  }
  for j < len(new) {
	fmt.Println(" add", new[j].SteamID)
	j++
  }
}

<kbd>playground example</kbd>