Ruby Method calls declared in class body

Question

I have just started learning ruby on rails and I have encountered code like below:

class Post < ActiveRecord::Base  validates_presence_of   :title  belongs_to :user end 

There are two method calls inside the class body. I have had a hard time finding any ruby documentation that describes how method calls from within a body of a class (but outside of any method) work. All the books I have, only describe how to define class and instance methods and how to call them from within other methods.

The questions I have are: How and when are these methods called? How are they defined? Are they mixins defined in some active record module?

Answer 1

The body of a class definition is an execution context for code just like any other. Code there is executed within the context of the class (meaning self is the class object, which is an instance of Class). You can have locals and instance variables (which will belong to the class object itself rather than instances of the class) and you can call any method that class object responds to. The code is run once the class definition block is finished.

In this case, ActiveRecord::Base defines the class methods validates_presence_of and belongs_to.

Answer 2

Yehuda Katz has a nice explanation of this on his blog. See point 4: Class Bodies Aren't Special.