Rails: callbacks from module

Question

I try to do this:

app/models/my_model.rb:

class MyModel <  ActiveRecord::Base
  include MyModule
  ...
end

lib/my_module.rb:

module MyModule
  before_destroy :my_func    #!

  def my_func
    ...
  end
end

but I get an error:

undefined method `before_destroy' for MyModule:Module

How can I correct it.

Also I'm new to ruby. What type has these "attributes": before_destroy, validates, has_many? Are they variables or methods or what? Thanks

Answer 1

before_destroy, validates, etc. are not attributes or anything like that. These are method calls.

In ruby, the body of a class is all executable code, meaning that each line of the class body is executed by the interpeter just like a method body would.

before_destroy :my_func is a usual ruby method call. The method that gets called is before_destroy, and it receives a symbol :my_func as an argument. This method is looked up in the class (or module) in the scope of which it is called.

So moving on to your question, I think now you should understand that when the interpreter loads your module

module MyModule
  before_destroy :my_func    #!

  def my_func
    ...
  end
end

it starts executing its body and searches for the method before_destroy in this module and cannot find one. What you want to do is call this method not on the module, but rather on the class where the module is included. For that we have a common idiom using the Module#included method:

module MyModule
  module InstanceMethods
    def my_func
      ...
    end
  end

  def self.included(base)
    base.send :include, InstanceMethods
    base.before_destroy :my_func
  end
end