The `posts` relation on `Topic` is not ordered. Using `Topic.posts.first`
is basically the same as asking for a random post, it will depend on DB
order. This breaks on Topic merge and split for example.
Additionally, a huge problem with that is that it forces active record down
a slow path. `Topic.posts.first` is extremely slow on giant topics, since
it has no default ordering it appears AR materializes the entire set prior
to doing `first`.
This commit also illustrates the importance of testing, initially I only
fixed the second instance of the problem in `post_validator.rb` but testing
revealed that the problem was repeated at the top of the file.
Longer term we should consider a larger change of default ordering the posts
relations so people do not fall down this trap anymore.
Since rspec-rails 3, the default installation creates two helper files:
* `spec_helper.rb`
* `rails_helper.rb`
`spec_helper.rb` is intended as a way of running specs that do not
require Rails, whereas `rails_helper.rb` loads Rails (as Discourse's
current `spec_helper.rb` does).
For more information:
https://www.relishapp.com/rspec/rspec-rails/docs/upgrade#default-helper-files
In this commit, I've simply replaced all instances of `spec_helper` with
`rails_helper`, and renamed the original `spec_helper.rb`.
This brings the Discourse project closer to the standard usage of RSpec
in a Rails app.
At present, every spec relies on loading Rails, but there are likely
many that don't need to. In a future pull request, I hope to introduce a
separate, minimal `spec_helper.rb` which can be used in tests which
don't rely on Rails.
