```ruby
class UserSerializer < ApplicationSerializer
  include MarkdownHelper

  attributes :avatar_url, :bio, :last_online, :staff, :url, :username

  def staff
    object.staff?
  end

  def url
    user_path(object)
  end

  def avatar_url
    object.avatar.url(140).to_s
  end

  def last_online
    object.last_request_at.iso8601 if object.last_request_at?
  end

  cached

  def cache_key
    [object]
  end
end
```

Remove caching from user serializer, last_request_at doesn't break the cache
user
```ruby
class UserSerializer < ApplicationSerializer
  include MarkdownHelper

  attributes :avatar_url, :bio, :last_online, :staff, :url, :username

  def staff
    object.staff?
  end

  def url
    user_path(object)
  end

  def avatar_url
    object.avatar.url(140).to_s
  end

  def last_online
    object.last_request_at.iso8601 if object.last_request_at?
  end
end
```
assistant