Testing polymorphic associations with RSpec

Question

I'm writing RSpec code for polymorphic associations. I have two testing ideas that

1. Use Factory Girl to build polymorphic associations
2. Use rails methods to build polymorphic associations.

Here are the pieces of the code I wrote (relevant codes are at the bottom):

1) link_spec.rb, creating the association with FactoryGirl.

describe "Book links" do
  let(:book) { FactoryGirl.create(:book) }
  let(:book_link) { FactoryGirl.create(:book_link, linkable: book) }

  subject{ book_link }

  it { should be_valid }

  it { should respond_to(:url) }
  it { should respond_to(:linkable) }
  its(:linkable) { should eq book }
end

2) link_spec.rb, creating the association through rails methods.

describe "Book links" do
  let(:book) { FactoryGirl.create(:book) }
  let(:link) { book.links.create(url: "http://example.com") }

  subject { link }

  it { should be_valid }

  it { should respond_to(:url) }
  it { should respond_to(:linkable) }
  its(:linkable) { should eq book }
end

I feel the latter is testing better than the former, but have no confidence. Or are they equivalent to each other?

---

book.rb

class Book < ActiveRecord::Base
  has_many :links, as: :linkable
end

link.rb

class Link < ActiveRecord::Base
  belongs_to :linkable, polymorphic: true
end

factories.rb

factory :book do
  title "Foo bar"
  author "John"
end

factory :book_link, class: "Link" do
  association :linkable, factory: :book
  url "http://examle.com"
end

Answer 1

I feel the latter is testing better than the former, but have no confidence.

If you intended to implicitly ask which was "better", then that question is not a good fit for StackOverflow.

Or are they equivalent to each other?

In the sense that the setup results in the same Link object being created, I would say that yes, they are equivalent. However, the second tests the association helper method set up for Book, which you may find desirable.