Background here.
In the above link, the following example is given:
class << self
def by_author(author)
where(:author_id => author.id)
end
end
Aside from that syntax being foreign to a beginner like me — I had always thought class methods were defined with def self.my_class_method — where can I find documentation about class methods in Ruby on Rails?
As far as I know, class methods are always called on the class itself (MyClass.my_class_method), but if class methods in Rails are chainable, it seems as though something else must be going on here!
Edit:
I suppose I sort of cheated by making that comment about the syntax for class methods. I'm really asking how Rails makes a class method chainable — I understand how method chaining works, but not how Rails can allow you to chain class methods without actually returning the class object itself after each "link" in the chain.
Class methods in Ruby are really just members of the singleton class, and doing class << self involves opening the singleton class directly and adding to it, removing the need to declare it in each method definition.
This article on Ruby singletons does a good job explaining.
As far as class methods being chainable, that isn't something specific to class methods, the second method call is simply called on the object returned from the first. For example:
bar = foo.do_something.do_more
is equivalent to:
tmp = foo.do_something
bar = tmp.do_more
In Rails, this chainability is most often used for building SQL queries (e.g., with where or order, etc.). This is achieved because each of these methods returns an ActiveRecord Relation.
The reason
foo.scoped.my_foo_class_method
works is because of ActiveRecord::Relation#method_missing doing the following:
elsif #klass.respond_to?(method)
scoping { #klass.send(method, *args, &block) }
Which checks if the ActiveRecord class responds to the method called, and if so, calls that.
Having class << self is also another way to define your methods so that you do not have to call "def self.my_method" or "def MyClass.my_method" for every single method that you are defining as a class method. Instead of calling
def self.my_method1
end
def self.my_method2
end
class << self
def my_method1
end
def my_method2
end
end
Cheers!
The following two bits of code are equivalent.
Using self.method:
class Hello
def self.world
puts "Hello, World!"
end
end
Using class << self:
class Hello
class << self
def world
puts "Hello, World!"
end
end
end
The only difference is readability, as well as the ease in refactoring.
The class << self technique is often used when metaprogramming.
There is another thread that explains this. class << self vs self.method with Ruby: what's better?
Related
This Ruby Style Guide tells that is better using self.method_name instead of class method_name. But Why?
class TestClass
# bad
class << self
def first_method
# body omitted
end
def second_method_etc
# body omitted
end
end
# good
def self.first_method
# body omitted
end
def self.second_method_etc
# body omitted
end
end
Are there performance issues?
class << self is good at keeping all of your class methods in the same block. If methods are being added in def self.method form then there's no guarantee (other than convention and wishful thinking) that there won't be an extra class method tucked away later in the file.
def self.method is good at explicitly stating that a method is a class method, whereas with class << self you have to go and find the container yourself.
Which of these is more important to you is a subjective decision, and also depends on things like how many other people are working on the code and what their preferences are.
Generally, class << self is used in metaprogramming to set the class as self for a prolonged period of time. If I'm trying to write 10 methods, I would use it like so:
METHOD_APPENDICES = [1...10]
class << self
METHOD_APPENDICES.each do |n|
define_method("method#{n}") { n }
end
end
This would create 10 methods (method1, method2, method3, etc.) that would just return the number. I would use class << self for clarity in this case because in metaprogramming self is crucial. Littering self. inside there would actually make things less readable.
If you're just defining class methods normally, stick to self.class_method_name because more people are likely to understand it. No need to bring in meta-syntax unless you expect your audience to understand it.
As noted above, both styles seem to be equivalent, however using class << self allows one to mark class methods as private or protected. For example:
class UsingDefSelf
def self.a; 'public class method'; end
private
def self.b; 'public class method!'; end
end
class UsingSingletonClass
class << self
def a; 'public class method'; end
private
def b; 'private class method'; end
end
end
private only affects instance methods. Using the singleton class, we are defining instance methods of that class, which turn into class methods of the containing class!
We can also mark class methods as private with def self:
class UsingDefSelf
def self.a; 'private class method'; end
def self.b; 'private class method!'; end
private_class_method :a, :b
# In Ruby 2.1 there is an alternative syntax
private_class_method def self.c; 'private class method!'; end
end
But we cannot mark them as protected, there is no protected_class_method. (However, since class is the only instance of its singletonclass, private class method and protected class methods are almost the same except their calling syntax is different.)
Also it is less easier than using class << self to mark private class methods, since you have to list all method names in private_class_method or prefix private_class_method to every private class method definition.
I assume that they think self.* is better because you can say for sure, it's a class or instance method, without having to scroll up and seaching this class << self string.
Whichever you want. Both are very clear for what you do. But I think of some recommendations for this.
When there're only one class method to define, Use def self.xxx. Because for defining only one method, increasing indent level probably become cluttering.
When there're more than one class method to define, Use class << self. Because writing def self.xxx, def self.yyy and def self.zzz is certainly repetition. Create a section for these methods.
When all methods in a class are class method, you can use module with module_function instead of class. This let you define module functions just use def xxx.
So far the question and answers only discuss these two options:
class MyClass
def self.method_name
..
end
end
class MyClass
class << self
def method_name
..
end
end
end
There's a third option to consider for class methods:
class MyClass
def MyClass.method_name
..
end
end
It's not popular and is more verbose, but it's the most explicit option.
It's also less confusing if you mix up self behaviour between Python and Ruby.
I'm working on rails app (3.2.11), and i'm implementing services as singleton, using the Ruby Library's one. I was trying to avoid calling :instance method every time I need it (SomethingService.instance.get(some_id)), solving my problem like this
class SomethingService
include Singleton
class << self
extend Forwardable
def_delegators :instance, *ShoppingListService.instance_methods(false)
end
end
So this solution was working perfectly fine but i've got a lot of services, and i don't want to add this code to all my classes! Instead i was trying to put in in a super class like this :
class ServiceBase
include Singleton
def self.inherited(subclass)
super(subclass) # needed because inherited is override by Singleton
class << subclass
extend Forwardable
def_delegators :instance, *self.instance_methods(false)
end
end
end
But this is giving me a stack level too deep error... Any ideas guys?
It's probably a better idea to use method_missing here. def_delegators is executed when the class is evaluated, and may happen before your methods are even defined, simply because you are inheriting from that base class.
You could try something like this instead, which forwards any undefined message onto the instance:
class ServiceBase
class << self
def method_missing(name, *args, &block)
instance.send(name, *args, &block)
end
end
end
It may look like a bit of a scattershot approach when compared to delegation. You could do a check to see if the instance method exists first, but I don't think that's necessary - you're simply transferring the undefined method handling to the instance rather than the class.
One final point/question - can you elaborate on what benefit you get from the Singleton class in the first place?
I've been studing Rails for a not such a long time up to now .... so if there are feel free to correct me
I see that there are two ways of defining methods in rails
def method_name(param)
def self.method_name(param)
The difference (as i understand) is that 1 is mainly used in controllers while 2 is used in models... but occasionaly i bump into methods in models that're defined like 1.
Could you explain to me the main difference of thease two methods?
Number 1. This defines a instance method, that can be used in instances of the model.
Number 2. This defines a class method, and can only be used by the class itself.
Example:
class Lol
def instance_method
end
def self.class_method
end
end
l = Lol.new
l.instance_method #=> This will work
l.class_method #=> This will give you an error
Lol.class_method #=> This will work
The method self.method_name defines the method on the class. Basically within the class definition think of self as referring to the class that is being defined. So when you say def self.method_name you are defining the method on the class itself.
class Foo
def method_name(param)
puts "Instance: #{param}"
end
def self.method_name(param)
puts "Class: #{param}"
end
end
> Foo.new.method_name("bar")
Instance: bar
> Foo.method_name("bar")
Class: bar
Let's say I have a model called Article:
class Article < ActiveRecord::Base
end
And then I have a class that is intended to add behavior to an article object (a decorator):
class ArticleDecorator
def format_title
end
end
If I wanted to extend behavior of an article object, I could make ArticleDecorator a module and then call article.extend(ArticleDecorator), but I'd prefer something like this:
article = ArticleDecorator.decorate(Article.top_articles.first) # for single object
or
articles = ArticleDecorator.decorate(Article.all) # for collection of objects
How would I go about implementing this decorate method?
What exactly do you want from decorate method? Should it simply add some new methods to passed objects or it should automatically wrap methods of these objects with corresponding format methods? And why do you want ArticleDecorator to be a class and not just a module?
Updated:
Seems like solution from nathanvda is what you need, but I'd suggest a bit cleaner version:
module ArticleDecorator
def format_title
"#{title} [decorated]"
end
def self.decorate(object_or_objects_to_decorate)
object_or_objects_to_decorate.tap do |objects|
Array(objects).each { |obj| obj.extend ArticleDecorator }
end
end
end
It does the same thing, but:
Avoids checking type of the arguments relying on Kernel#Array method.
Calls Object#extend directly (it's a public method so there's no need in invoking it through send).
Object#extend includes only instance methods so we can put them right in ArticleDecorator without wrapping them with another module.
May I propose a solution which is not using Module mixins and thereby granting you more flexibility. For example, using a solution a bit more like the traditional GoF decorator, you can unwrap your Article (you can't remove a mixin if it is applied once) and it even allows you to exchange the wrapped Article for another one in runtime.
Here is my code:
class ArticleDecorator < BasicObject
def self.[](instance_or_array)
if instance_or_array.respond_to?(:to_a)
instance_or_array.map {|instance| new(instance) }
else
new(instance_or_array)
end
end
attr_accessor :wrapped_article
def initialize(wrapped_article)
#wrapped_article = wrapped_article
end
def format_title
#wrapped_article.title.upcase
end
protected
def method_missing(method, *arguments)
#wrapped_article.method(method).call(*arguments)
end
end
You can now extend a single Article by calling
extended_article = ArticleDecorator[article]
or multiple articles by calling
articles = [article_a, article_b]
extended_articles = ArticleDecorator[articles]
You can regain the original Article by calling
extended_article.wrapped_article
Or you can exchange the wrapped Article inside like this
extended_article = ArticleDecorator[article_a]
extended_article.format_title
# => "FIRST"
extended_article.wrapped_article = article_b
extended_article.format_title
# => "SECOND"
Because the ArticleDecorator extends the BasicObject class, which has almost no methods already defined, even things like #class and #object_id stay the same for the wrapped item:
article.object_id
# => 123
extended_article = ArticleDecorator[article]
extended_article.object_id
# => 123
Notice though that BasicObject exists only in Ruby 1.9 and above.
You'd extend the article class instance, call alias_method, and point it at whatever method you want (although it sounds like a module, not a class, at least right now). The new version gets the return value and processes it like normal.
In your case, sounds like you want to match up things like "format_.*" to their respective property getters.
Which part is tripping you up?
module ArticleDecorator
def format_title
"Title: #{title}"
end
end
article = Article.top_articles.first.extend(ArticleDecorator) # for single object
Should work fine.
articles = Article.all.extend(ArticleDecorator)
May also work depending on ActiveRecord support for extending a set of objects.
You may also consider using ActiveSupport::Concern.
This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
class << self idiom in Ruby
Can someone please explain class << self to me?
I would like to know what does class << self statement mean in a model class? And how does the statement inside it differ from those outside from it. For example:
class Post < ActiveRecord::Base
class << self
def search(q)
# search from DB
end
end
def search2(qq)
# search from DB
end
end
What does class << self mean?
What are the differences between method search(q) and search2(qq) ?
That is the same as
class Post < ActiveRecord::Base
def self.search(q)
# Class Level Method
# search from DB
end
def search2(qq)
# Instance Level Method
# search from DB
end
end
Class methods work on the class (e.g. Post), instance methods works on instances of that class (e.g. Post.new)
Some people like the class << self; code; end; way because it keeps all class level methods in a nice block and in one place.
Others like to prefix each method with self. to explicitly know that is a class method not an instance method. It's a matter of style and how you code. If you put all class methods in a block like class << self, and this block is long enough, the class << self line might be out of your editor view making it difficult to know that you are in the class instance block.
On the other hand, prefixing each method with self. and intermixing those with instance methods is also a bad idea, how do you know all the class methods while reading your code.
Pick an idiom which you prefer for your own code base but if you work on an open source project or you collaborate on someone else's code, use their code formatting rule.
It creates class methods as opposed to instance methods. It's the same as doing def self.search(q). Those two methods would be called like:
Post.search(q)
Post.new.search(qq)
search2 is an instance-method while search is a class-method. The class << self syntax enables you to group class methods below. There are three ways of defining class methods in Ruby:
class MyClass
def self.method
# do sth.
end
def MyClass.method2
# do sth.
end
class << self
def method3
# do sth.
end
def another_class_method
# do sth.
end
end
end
Class-methods are called on a class, not on an instance. It is personal preference which of the three idioms you want to use. I prefer def self.method for no particular reason.