This slideshow covers how one might implement the Null Object Pattern, that is, a system for handling potentially null objects in your code in a way that allows for more maintainability.

1. The
Null Object Pattern

2. How nils are often handled
@account.most_recent_event.nil ? 'no event' : @account.most_recent_event.name

3. How nils are often handled
@account.most_recent_event.nil ? 'no event' : @account.most_recent_event.name
Not so good

4. Step 1 - DRY It Up
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.nil? 'no event' : most_recent_event.name
end
!
end
!
!

5. Step 1 - DRY It Up
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.nil? 'no event' : most_recent_event.name
end
!
end
!
!
Still have ‘type-checking’ duplication

6. Step 1 - DRY It Up
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.nil? 'no event' : most_recent_event.name
end
!
def most_recent_event_date
most_recent_event.nil? 'no event' : most_recent_event.date
end
end
!
!
Still have ‘type-checking’ duplication

7. Step 1 - DRY It Up
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.nil? 'no event' : most_recent_event.name
end
!
def most_recent_event_date
most_recent_event.nil? 'no event' : most_recent_event.date
end
end
!
!
Tell, Don’t Ask

8. Step 2 - Tell Don’t Ask
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.name
end
!
end
!
!

9. Step 2 - Tell Don’t Ask
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.name
end
!
end
!
!
#=> NoMethodError: undefined method `name' for nil:NilClass

10. Step 2 - Tell Don’t Ask
class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.name
end
!
end
!
!
#=> NoMethodError: undefined method `name' for nil:NilClass

11. class Account < ActiveRecord::Base
!
belongs_to :account
!
def most_recent_event
events.last
end
!
end
!
!

12. class Account < ActiveRecord::Base
!
belongs_to :account
!
def self.most_recent_event
events.last || <SOMETHING WHICH RESPONDS TO #name>
!
end
!
end
!
!

13. Step 3 - Create an Interface
class NullEvent
!
def name
'no event'
end
!
end

14. class Event < ActiveRecord::Base
!
belongs_to :account
!
def self.most_recent
events.last || <SOMETHING WHICH RESPONDS TO #name>
end
!
end
!
!

15. class Event < ActiveRecord::Base
!
belongs_to :account
!
def self.most_recent
events.last || NullEvent.new
end
!
end
!
!

16. class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.nil? 'no event' : @account.most_recent_event.name
end
!
end
!
!
Before

17. class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.name
end
!
end
!
!
After

18. class Account < ActiveRecord::Base
!
has_many :events
!
def most_recent_event_name
most_recent_event.name
end
!
def most_recent_event_date
most_recent_event.date
end
end
!
!
After

19. Step 3 - Create an Interface
class NullEvent
!
def name
'no name'
end
!
def date
'no date'
end
!
end

20. Already Used at PP
!
class DummyGuest
def state
'initialized'
end
!
def total_guests_allowed
1
end
!
...etc.
end

21. Benefits

22. * encapsulation
Benefits

23. * encapsulation
Benefits
* “tell, don’t ask” style

24. Problems

25. Problems
*parallel interface maintenance

26. Problems
*parallel interface maintenance
*confusing to those not expecting it