method invocation in class definition?

Question

class Person < ActiveRecord::Base
  validates :terms_of_service, :acceptance => true
end

In the above, what is validates from a

Answer 1

In Ruby, class declarations are just chunks of code, executed in order.

It's important to remember that inside a class definition, self points to the class itself. validates is a class method of ActiveRecord. As the class is being defined, code in the definition is executed. The validates method resolves to a class method of ActiveRecord, so is called during class definition.

In your Person example, it will only print once, because you only define the class once.

Consider the following:

class Foo
  def self.validates_nothing(sym)
    (@@syms ||= []) << sym
    puts "!!! Here there be logic"
  end

  def validate
    @@syms.each { |s| puts s }
  end
end

This defines a class with a class method validates_nothing, and an instance method, validate. validates_nothing just gathers whatever arguments are given it, validate just dumps them out.

class Bar < Foo
  validates_nothing :anything
  validates_nothing :at_all
end

This defines a subclass. Note that when the class method validates_nothing is called, it prints:

Here there be logic
Here there be logic

If we create a new bar and call validate, we get the expected output:

> Bar.new.validate
!!!anything
!!!at_all