如何测试数据库记录是否按 id 排序？

Question

I'm testing a JSON API. And up until recently I assumed the records are returned ordered by id. But when Posts.all turned into Posts.where(user_id: params[:user_id]) , it broke the following test:

it 'takes "limit" param' do
  post1 = create :post
  post2 = create :post, user_id: post1.user_id
  get user_posts_path(post1.user_id, limit: 1)
  expect(response.body).to be_json_eql([{id: post1.id}].to_json).including(only: :id)
end

I can probably add the following test:

it 'returns records ordered by id' do
  post1 = create :post
  post2 = create :post, user_id: post1.user_id
  get user_posts_path(post1.user_id)
  expect(response.body).to be_json_eql([{id: post1.id}, {id: post2.id}].to_json).including(only: :id)
end

but it succeeds without adding .order(:id) to the controller. I'd like to make sure it's there. What do I do?

Answer 1

The order is not guaranteed, so while you might get the behaviour of the results being ordered by id, it's not guaranteed, and you should not rely on it.

See " Why is SSMS inserting new rows at the top of a table not the bottom? "

Also, as it mostly returns by id, your test will mostly succeed without the order set but it tells you nothing and is bad practice.

So, are you really asking if I do something that's not safe, how can I write a test to check if something bad happens as result?

I'm not sure why you would do that. You've got a potential intermittent bug that will appear when the database decides it wants to, on any environment. Even if it doesn't happen in your testing environment it may happen in production, so it tells you nothing. Remove the bug by adding an order, then your test should pass through a known behaviour.