如何测试模型实例方法?

我在实现Rspec时遇到了一些问题。我有三个模型; Post ,则, Tagging 和 Tag 。

应用程序/型号/标签。rb

class Tag < ApplicationRecord
  # associations
  has_many :taggings
  has_many :posts, through: :taggings

  # validations
  validates :name, presence: true, uniqueness: { case_sensitive: false }

  # returns a list of posts that are belonging to tag.
  def posts
      ...
  end
end

我能够为关联和验证编写规范,但仍坚持为的实例方法编写规范 def posts ... end .有人能简要解释一下如何编写此规范吗?我是Rspec的新手,所以请容忍我。

规格/型号/tag\u spec.rb

require 'rails_helper'

RSpec.describe Tag, type: :model do
  describe "Associations" do
    it { should have_many(:posts).through(:taggings) }
  end

  describe "Validations" do
    subject { FactoryBot.create(:tag) }

    it { should validate_presence_of(:name) }
    it { should validate_uniqueness_of(:name).case_insensitive }
  end

  describe "#posts" do
    # need help
  end

end