Collection count is reset when model saved

[starhoshi]

export class Cart extends Pring.Base {
  @property cartSKUs: Pring.NestedCollection<CartSKU> = new Pring.NestedCollection(this)
}

export class CartSKU extends Pring.Base {
  @property quantity: number
}

    const cart = new Firebase.Cart()

    const cartSKU1 = new Firebase.CartSKU()
    cartSKU1.quantity = 1

    cart.cartSKUs.insert(cartSKU1)
    console.log(cart.cartSKUs.count()) // => 1

    await cart.save()
    console.log(cart.cartSKUs.count()) // => 0