ActiveRecord enum behaves differently between rails 4 and rails 5

Summary

ActiveRecord::Enum behaves differently between rails 4 and rails 5 as @vsizov reported at https://gitlab.com/gitlab-org/gitlab-ce/commit/6c0d5618050691b5e06b9f576e45bd7ccfaa7c46#note_39933822.

Specifically, rails 5 enum can't evaluate correctly when it has nil value. For example,

enum failure_reason: {
  unknown_failure: nil,
  script_failure: 1,
  api_failure: 2,
  stuck_or_timeout_failure: 3,
  runner_system_failure: 4
}

commit_status.unknown_failure? returns false, even if it's true in rails 4.

This has been patched at https://gitlab.com/gitlab-org/gitlab-ce/commit/6c0d5618050691b5e06b9f576e45bd7ccfaa7c46#note_39933822 as

  # This can be considered as a hack as nil value is not documented (as a enum value)
  # in Rails Enum documentation and it behave defferently in rails 4 and rails 5.
  # In particular, unknown_failure? always returns false even if value of failure_reason is nil.
  # This method just overwrites the autogenerated unknown_failure? enum method.
  def unknown_failure?
    failure_reason.nil?
  end

Ideally, we want to fix rails 5 itself or something better solution.

Also, we have some enum which have nil. We should take care of them as well.

/cc @ayufan @bikebilly