Rspec before(:each)起作用,before(:all)不起作用

require 'rails_helper'

RSpec.describe ProductCategory, type: :model do
    before(:each) do 
        @product_category = create(:product_category)
    end

  context "validations" do 
    it "should have valid factory" do
        expect(@product_category).to be_valid
    end

    it "should have unique name" do 
        product_category_new = build(:product_category, name: @product_category.name)
        expect(product_category_new.save).to be false
    end
  end
end

该规范运行良好,但当我使用before(:all)而不是before(:each)时,第二个示例失败-
expected false got true 我知道before(:all)和before(:each)之间的区别,但我找不到第二个示例在before(:all)中失败的确切原因