From: Gavin Kistner <gavin@...>
Date: 2007-07-25T04:30:14+09:00
Subject: Re: Bidirectionnal relation

dohzya wrote:
> Hi everyone,
> I have a problem with bidirectional relations : how to implements it ?

How about:
class Female
  attr_accessor :mother
  def initialize( name )
    @name = name
    @daughters = []
  end
  def daughters
    @daughters.dup.freeze
  end
  def mother=( mom )
    @mother = mom
    unless mom.daughters.include?( self )
      mom.add_daughter( self )
    end
  end
  def add_daughter( girl )
    @daughters << girl unless @daughters.include?( girl )
    girl.mother = self
  end
  def inspect
    "<Female '#{@name}'>"
  end
end

a = Female.new( 'Strami' )
b = Female.new( 'Lisa' )
c = Female.new( 'Imogen' )

b.mother = a
p a.daughters.include?( b )
#=> true

b.daughters << c
#=> Error: can't modify frozen array (TypeError)

b.add_daughter c

p a.daughters
#=> [<Female 'Lisa'>]

p b.daughters
#=> [<Female 'Imogen'>]

p c.mother
#=> <Female 'Lisa'>
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