Box data structure in Go

Would the following be considered idiomatic Go code?

My main concerns are:

-
The use of maps for this purpose. I basically want the data structure to be completely dynamic in size and have easy access to elements by id. This is because I will want to retrieve specific items on demand lots of times, so I don't want to loop each time.

-
The check for the map element

-
My use of pointers

-
General flow and structure

package main

    import (
        "fmt"
    )

    type BoxItem struct {
        Id  int
        Qty int
    }

    func NewBox() *Box {
        return &Box{make(map[int]*BoxItem), 0}
    }

    type Box struct {
        BoxItems map[int]*BoxItem
    }

    func (this *Box) AddBoxItem(id int, qty int) *Box {

        i, ok := this.BoxItems[id]

        if ok {
            i.Qty += qty
        } else {
            boxItem := &BoxItem{id, qty}
            this.BoxItems[id] = boxItem
        }

        return this
    }

    func main() {

        box := NewBox()
        box.AddBoxItem(1, 1)

        fmt.Printf("There are %d items in the box", len(box.BoxItems))
    }

Instead of

i, ok := this.BoxItems[id]
if ok {
    i.Qty += qty
}

a more idiomatic usage is

if i, ok := this.BoxItems[id]; ok {
    i.Qty += qty
}

Calling the receiver this doesn't seem like a good practice to me. Calling it box would be clearer.

You seem to want to make your method chainable. I'm not sure this is a frequent practice in Go but it's probably more a matter of style and experience than a question of idiomatic code or not.

The rest could be condensed a little but that's not a matter of idiom. I have nothing to say regarding your use of pointers.

StackExchange Code Review Q#29524, answer score: 4