Dirty checking is partially broken on Rails 5.2

[atavistock]

On Rails 5.1 when a field was changed you would get the key: [FileUpload..., FileUpload] (or whatever Upload) objects when calling changes. In Rails 5.2, changes its only giving back nils for the before and after change information, as in key: [nil, nil]. This is a fairly big problem if you're trying to introspect and make decisions about how the content might have changed.

---

Test code:

m = ActiveRecord::Migration.new 
m.create_table :lorems do |t|
  t.string :image
end

class Lorem < ActiveRecord::Base
  mount_uploader :image, FileUploader
end

l = Lorem.new
l.image = File.new('/etc/paths')
puts l.changes

Results under Rails 5.1

{"image"=>[#<FileUploader:0x00007fd5ec9d3ce8 @model=#<Lorem id: nil, image: nil>, @mounted_as=:image, @file=nil, @filename=nil, @cache_id=nil, @versions=nil>, #<FileUploader:0x00007fd5ec9d3c20 @model=#<Lorem id: nil, image: nil>, @mounted_as=:image, @file=#<CarrierWave::SanitizedFile:0x00007fd5ec9d1b78 @file="/Users/aaron/src/anomalie/anomalie/public/uploads/tmp/1562175246-39911-0001-8025/paths", @original_filename="paths", @content_type="", @content=nil>, @filename="paths", @cache_id="1562175246-39911-0001-8025", @versions={}, @original_filename="paths", @cache_storage=#<CarrierWave::Storage::File:0x00007fd5ec9d0700 @uploader=#<FileUploader:0x00007fd5ec9d3c20 ...>, @cache_called=nil>>]}

Results under Rails 5.2

{"image"=>[nil, nil]}