Вызовы метода Ruby объявляются в теле класса

Тело определения класса - это контекст выполнения для кода, как и любого другого. Код там выполняется в контексте класса (то есть self - это объект класса, который является экземпляром класса). У вас могут быть локальные переменные и переменные экземпляра (которые будут принадлежать самому объекту класса, а не экземплярам класса), и вы можете вызвать любой метод, на который отвечает объект класса. Код запускается после завершения блока определения класса.

В этом случае ActiveRecord ::

Re: How and when are these methods called?

[They're called when the class is loaded. You can put a breakpoint in one of the methods and see that it is called as part of the startup of your rails project.]

How are they defined?

[They're class methods. Since this is ruby, they could be defined in a number of ways.]

Are they mixins defined in some active record module?

[In this case,

validates_presence_of is defined in vendor/rails/activerecord/lib/active_record/validations.rb

belongs_to is defined in vendor/rails/activerecord/lib/active_record/associations.rb

ActiveRecord is a big system, includes many mixins, modules, etc.

Note, to see where the methods are defined, I use http://www.gotapi.com/rubyrails for each method, see the "Show Source" link at the bottom of the definition.

]

У Иегуды Каца есть хорошее объяснение этого в своем блоге. См. Пункт 4: Классовые тела не особенные .

These are class methods or 'singleton' methods. One you should be familiar with is attr_accessor. We can implement something like it in a test class.

class Klass
  def self.add_getter_and_setter(symbol)
    module_eval "def #{symbol}; @#{symbol}; end"
    module_eval "def #{symbol}=(val); @#{symbol} = val; end"
  end
end

class Person < Klass
  add_getter_and_setter :name
  add_getter_and_setter :phone
end

person = Person.new
person.name = 'John Smith'
person.phone = '555-2344'
person # returns <Person:0x28744 @name="John Smith", @phone="555-2344">

In the above example we created the class method with 'def self.add_getter_and_setter' but this is not the only way.

class Klass
  class << self # opens the singleton class
    def add_getter_and_setter(symbol)  # note we dont specify self as it is already within the context of the singleton class
      ..
    end
  end
end

Using extend. Module#extend is a method that extends a class with class methods likewise the method Module#include includes a class with instance methods.

class Klass
  extend(Module.new do
    def add_getter_and_setter(symbol)
      ..
    end
  end)
end

If Klass has already been defined we can reopen it to add class methods

class Klass
end

def Klass.add_getter_and_setter(symbol)
  ..
end

# or 

class << Klass
  def add_getter_and_setter(symbol)
    ..
  end
end

Well those are a few ways I know how to do this so if you see different syntax just realize its all doing the same thing.

Note: in rails a common class method we all use is 'find'. It is run directly off the Model class.

person = Person.find(1) # finds a person with id:1

What you are seeing are class level methods for an ActiveRecord object. To write your own methods that perform like that you would write them as a plugin and then include them into ActiveRecord by re-opening the class definition. The Ruby on Rails guide to creating a plugin:

http://guides.rubyonrails.org/plugins.html

Covers how one would write such a plugin / class-level methods. Its a good document on how to wrap your head around what those kinds of methods mean and how they interact with instances.