Question

I am developing a Rails application and would like to understand when to use self.for.

Here is the code of a method that I would like to fully understand. If it is possible I would like to have an alternative to this code so it would make things more clear.

def self.for(facebook_id)
  User.create_by_facebook_id(facebook_id)
end

Answer 1

self refers to the current object.

Within a class, self is used to define a class-level method.

class Foo
  def self.for(facebook_id)
    User.create_by_facebook_id(facebook_id)
  end
end

defines a class method for in class Foo. It is invoked:

Foo.for(facebook_id)

You can google for class methods to learn more.

It could be that a part of Rails or a plugin/gem is expecting that some classes will have a class method “for” More context would be helpful in this regard.

What the method is doing

As is common for class methods, it is creating an instance of a class. For example, the ActiveRecord class has a class method “create” which attempts to create an instance of the model class that has been stored in the database. Thus User.create will return an instance of the User class that has been stored in the database.

In your example code, it is calling a class method “create_by_facebook_id” that is provided by the User class in the application.

Looks like the “for” method is being used for information hiding since all it’s doing is making another method call (to User.create_by_facebook)

Added:

By the way, the default return value from Ruby methods is the value of the last statement. So your example method will return the user instance newly created from the supplied facebook_id.