add attribute at runtime

June 16, 2009 at 6:23 am (Model) (attr_accessor, class_eval, rails, ROR, ruby, ruby on rails, runtime)

I’m all excited abt this post.

Coz this is something i learnt just now, and consider it my best learning till now in RUBY.

Q: How to add attributes to a class at runtime?

A:

class Class
def new_attr (attr_name)

class_eval {attr_accessor :"#{attr_name}"}
end
end


class_eval : Evaluates a string or a block in the context of the receiver, here the class.

Explanation:

1) We defined a new method to the CLASS called new_attr, which is intended to add new attributes at runtime.

2) This method needs the name of the new attribute to be passed.

3) In the definition, it calls class_eval, which means “DO SOMETHING AT THE CLASS LEVEL”

4) What to do is specified in the block after the class_eval, which says : attr_accessor “#{attr_name}”, which will become attr_accessor :name when we call new_attr(‘name’).

5) We know that attr_accessor is rails syntactic sugar to create the a specific instance variable, and its set/get (read/write) methods. So here, when we call new_attr(‘name’), its creating a new instance variable (@name) and 2 new methods ‘name” and “name=”.

Try this;

class Person; end;

This creates a bare class Person.

p = Person.new

Try, p.name, it throws an error cursing “Undefined method name”.

Its fairly expected coz, Person has no attribute called name.

Now, say Person.new_attr(‘name’)

now try, p.name, it will not curse you.

To check, try Person.singleton_methods.include?(“name”)

This says TRUE.

Hope this article comes handy.