in rspec, why do I need the '[]' in expect().to eq([])? - ruby-on-rails

Question

Rails app, writing specs:
RSpec.describe AdvertisementsController, :type => :controller do
let(:my_ad) { Advertisement.create!(title: 'title', copy: 'copy text', price: 10)}
describe 'GET #index' do
...
...
it 'renders my_ad' do
get :index, {id: my_ad.id}
expect(assigns[:advertisements]).to eq(my_ad)
end
end
...
...
end
I wrote the above, which gave the error below.
1) AdvertisementsController GET #index renders my_ad
Failure/Error: expect(assigns[:advertisements]).to eq(my_ad)
expected: #<Advertisement id: 1, title: "title", copy: "copy text", price: 10, created_at: "2016-02-26 02:39:20", updated_at: "2016-02-26 02:39:20">
got: #<ActiveRecord::Relation [#<Advertisement id: 1, title: "title", copy: "copy text", price: 10, created_at: "2016-02-26 02:39:20", updated_at: "2016-02-26 02:39:20">]>
(compared using ==)
Diff:
## -1,8 +1,8 ##
-#<Advertisement:0x007ff6995e19f0
- id: 1,
- title: "title",
- copy: "copy text",
- price: 10,
- created_at: Fri, 26 Feb 2016 02:39:20 UTC +00:00,
- updated_at: Fri, 26 Feb 2016 02:39:20 UTC +00:00>
+[#<Advertisement:0x007ff6994f9560
+ id: 1,
+ title: "title",
+ copy: "copy text",
+ price: 10,
+ created_at: Fri, 26 Feb 2016 02:39:20 UTC +00:00,
+ updated_at: Fri, 26 Feb 2016 02:39:20 UTC +00:00>]
With this code, the test will pass. All it seems to do is add [..] around the variable, as such (| & ^ to emphasize location):
Why does that work?
RSpec.describe AdvertisementsController, :type => :controller do
let(:my_ad) { Advertisement.create!(title: 'title', copy: 'copy text', price: 10)}
describe 'GET #index' do
...
...
it 'renders my_ad' do
get :index, {id: my_ad.id}
expect(assigns[:advertisements]).to eq([my_ad])
end ^ ^
end | |
...
...
end
of note I can see that the objects have different identifying id's, so I think that's part of the reason and the [..]'s either ignore the mismatch or something, but I'd like to understand it.
-#<Advertisement:0x007ff6995e19f0 vs . +[#<Advertisement:0x007ff6994f9560

Answer 1

[] is Ruby syntax for an array. So your test is expecting to assign an array of advertisements (or in this case, something that behaves like an array, such as an ActiveRecord::Relation), containing just one element, my_ad.
Your code also sounds quite strange in that you're providing an ID to the index action, expecting only that record to be returned. Index actions are for listing out groups of records - a show action is for showing the details of a single record.