From: Stefano Crocco <stefano.crocco@...>
Date: 2008-03-16T04:21:15+09:00
Subject: Re: How to iterate through instance attribute names (attr_accessor)

On Saturday 15 March 2008, me wrote:
> Say, I have a class:
>
> class Event
>   attr_accessor	:a,
>
>                         :b,
>                         :c,
>                         :d
>
>   def initialize()
>   end
>
>   def validate
>     # validate all attributes at once here
>   end
>
>   protected
>
>   def validate_a
>     # a can be any number from 1-10 if b = 'Y' and c is blank
>   end
>
>   def validate_b
>     # b can be 'Y' or 'N'
>   end
>
>   def validate_c
>   end
>
>   def validate_d
>   end
>
> end
>
> And in method validate, I'd like to iterate through all the attributes
> and run their respective validating method.  Is there a way for me to
> be able to iterate through my list of attr_accessor symbols and
> reference their names to know which method to call?
>
> Thanks,
>
> -Al

You must understand that all attr_accessor does is create a couple of methods: 
a setter and a getter. For example

attr_accessor :a

creates the two methods a and a=, which are more or less equivalent to the
following hand-written methods:

def a
  @a
end

def a= value
  @a = value
end

Once defined, the methods created by attr_accessor can't be distinguished from 
the other instance methods of the class. This means that you can't iterate on 
the methods themselves. What you can do is iterate on the instance variables 
methods created this way refer to. For example, your 'validate' method could
be something like:

def validate
  instance_variables.each{|v| send "validate_#{v.sub('@','')}"}
end

instance_variables is a method which returns an array with the names of all
the instance variables of the object. The method send, instead, calls the
method with the name passed as its first argument (in this case, 'validate_a', 
'validate_b' and so on. v.sub is needed because the names of the instance 
variables contain the initial '@').

The code above works provided there's a validate_ method for each instance
variable. If this is not the case, you can enclose the call to send in a
begin/rescue block:

def validate
  instance_variables.each do |v|
    begin send "validate_#{v.sub('@','')}"
    rescue NoMethodError
    end
  end
end

Of course, you can also keep an array of the names of those instance variables 
for which a validation method exists and use the following code:

def validate
  [:a, :b, :c, :d].each{|v| send "validate_#{v}"}
end

I hope this helps

Stefano