This is a follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19288 The problem is that couple of models currently use `enum` where a key is mapped to `nil` value. `nil` value is not supported in Rails 5. Because changing mapping from `nil` to an `integer` may be tricky (and will probably require a DB migration), as a short-term solution, gitlab-ce!19288 adds a new helper method `enum_with_nil` which enables `nil` value for enums. But it would be much better to align out `enum` definitions with default rails behavior and we should rather migrate `nil` values to an `integer`.