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I often find myself resorting to eval when writing class methods for metaprogramming, simply because define_method hasn’t penetrated my brain basket yet. No more!

Here goes Florian GroBB changing Ruby as I know it—yet again—with his implementation of an eval-less has_many class method:

 class Meta
   def self.has_many( sym )
     ivar = :"@#{ sym }" 
     define_method( sym.to_sym ) do
       new_value = instance_variable_get( ivar ) || []
       instance_variable_set( ivar, new_value )
       new_value
     end
   end
 end

Even though has_many is a sorta poor name (we’re not talking about databases here), this method acts like an attr method, but the attribute is always an Array.

 class Person < Meta
   attr_accessor :name
   has_many :friends
   def initialize( name ); @name = name; end
 end

 >> fred = Person.new( "Fred" )
 >> joe = Person.new( "Joe" )
 >> teagle = Person.new( "T. Eagle" )
 >> teagle.friends << fred
 >> teagle.friends << joe
 => [#, #]