Using Null Object pattern with Ruby on Rails 1
While back ago I’ve released plugin to implement Null Object Pattern. And I’ve decided to write about it. Mostly because people are not using it or don’t know it.
What it does?
Instead of using a null reference to convey absence of an object (for instance, a non-existent customer), one uses an object which implements the expected interface, but whose method body is empty. The advantage of this approach over a working default implementation is that a Null Object is very predictable and has no side effects.
Why should I use it?
- You will not have to prevent your code from having null reference. For example you will not have to do this:
1print post.user.nil? ? 'Null user' : post.user.name
With Null Object pattern you write just print post.user.name and you’ll get default value for Null object defined for this attribute.
- It’s very handy if you’re doing some kind of reporting that you don’t have to deal with null reference and you have object (in SQL row reference) instead.
To install the plugin
1 | ruby script/plugin install git://github.com/lacomartincik/acts_as_nullobject.git |
Let’s have a look how to use it
First you’ll need to define model to acts as null object:
1 2 3 | class User < ActiveRecord::Base acts_as_nullobject :login => 'lacomartincik', :name => 'Ladislav Martincik' end |
Than whenever you call on User class method null_object you will get instance of NullUser class with default attributes defined in User:
1 2 | >> User.null_object => NullUser... |
Now if you define relationship between say Project and User like this:
1 2 3 | class Project < ActiveRecord::Base belongs_to_with_nullobject :user end |
Than whenever you create new Project object with null reference to User you will get NullUser instance instead of nil:
1 2 3 | >> p = Project.new >> p.user => NullUser |
The project is available as plugin on http://github.com/lacomartincik/acts_as_nullobject.
Enjoy! Any suggestions, bug reports and comments welcomed!
Update: I had to rename plugin name from acts-as-nullobject to acts_as_nullobject otherwise it’s not going to work in Rails!