我的模型中有一个方法，如果多个项目具有相同的variant_id，则按variant_id对项目进行分组，然后合并它们并添加它们的数量。
我的模特：

class ShoppingCart < ApplicationRecord
  belongs_to :company
  belongs_to :user
  has_many :items, class_name: "ShoppingCartItem", dependent: :destroy
  accepts_nested_attributes_for :items, reject_if: proc { |attributes| attributes['quantity'].blank? }

  before_validation :remove_invalid_items
  before_validation :merge_items

  def merge_items
    self.items = items.group_by { |i| i[:variant_id] }.map do |variant_id, is|
      quantity_sum = is.sum { |i| i[:quantity] }
      ShoppingCartItem.new(variant_id: variant_id, quantity: quantity_sum)
    end
  end
end

当我手动尝试时，这个方法工作得很好，但是在我的测试中，rspec似乎忽略了这个方法
我的测试：

require 'rails_helper'

RSpec.describe ShoppingCart, type: :model do
  describe "associations" do
    it { is_expected.to belong_to(:company) }
    it { is_expected.to belong_to(:user) }
    it { is_expected.to have_many(:items) }
  end

  describe "merge_items" do
    let(:shopping_cart) { create(:shopping_cart) }

    it "merge items if same variant_id" do
        existing_item = create(:shopping_cart_item, shopping_cart: shopping_cart, variant_id: "same variant_id", quantity: 1)
        item = create(:shopping_cart_item, shopping_cart: shopping_cart, variant_id: "same variant_id", quantity: 1)

        expect(shopping_cart.reload.items.count).to eq(1)
    end

    it "not merge items if variant_id different" do
      existing_item = create(:shopping_cart_item, shopping_cart: shopping_cart, variant_id: "variant_id", quantity: 1)
      item = create(:shopping_cart_item, shopping_cart: shopping_cart, variant_id: "different variant_id", quantity: 1)

      expect(shopping_cart.reload.items.count).to eq(2)
    end
  end
end

测试输出：
失效/错误：expect（购物车.重载.商品.计数）. to eq（1）

expected: 1
        got: 2