expect(account.shopping_cart).to have_exactly(3).items

require 'rspec/collection_matchers'

it 'matches number of items in a collection' do
  expect([1,2,3]).to have_at_least(3).items
end

class Cart
  def initialize(*products)
    @products = products
  end
  attr_reader :products
end
it 'matches number of items returned from a method' do
  cart = Cart.new('product a', 'product b')
  expect(cart).to have_at_most(2).products
end

expect(collection).to have(n).items
expect(collection).to have_exactly(n).items
expect(collection).to have_at_most(n).items
expect(collection).to have_at_least(n).items
expect(collection).to have(:no).items