Adding sub items qty's increase the wrong item

[lukepolo]

add x1 (shows qty_x = 1)
add x1 with sub y1 (shows qty_xy = 1)
add x1 with sub y1 again and all of a sudden qty_x = 2
qty_xy should = 2

The solution would be to change the qty of x1. And re-add x2 as a new item with the sub item (which is no problem). The problem is x2 is no longer x2. For instance

    $x1 = $this->addItem();
    // $x2 is really just $x1 now since we increased the qty to 2
    $x2 = $this->addItem();
    // $x2 is now with the sub item
    $x2->addSubItem([
        'size' => 'XXL',
        'price' => 2.50
    ]);
    // and $x2 should be its own item but its attached to x1

The TEST :

public function testSubItemQtys()
{
    $item_1 = $this->addItem();

    $item_2 = $this->addItem();

    $subItem = $item_2->addSubItem([
        'size' => 'XXL',
        'price' => 2.50
    ]);

    $this->assertEquals(1, $item_1->qty);
    $this->assertEquals(1, $item_2->qty);

    $item_3 = $this->addItem();

    $subItem = $item_3->addSubItem([
        'size' => 'XXL',
        'price' => 2.50
    ]);

    $this->assertEquals(1, $item_2->qty);
    $this->assertEquals(2, $item_2->qty);
}

This test fails because of these issues.