From "Well Grounded Rubyist" by David A. Black, Listing 4.11 Simple usage of the Person Class (page 107), Listing 4.12 Implementation of the main logic of the Person class (page 108), Listing 4.13 Full implementation of Person.method_missing (page 109),

4.11_code.rb

j = Person.new("Julia")
p = Person.new("Peter")
g = Person.new("Gabriel")
r = Person.new("Rebecca")

j.has_friends(p)
j.has_friends(r)
g.has_friends(p)
r.has_hobbies("Stargazing")

Person.all_with_friends(p).each do |person|
	puts "#{person.name} is friends with #{p.name}"
end

Person.all_with_hobbies("Stargazing").each do |person|
	puts "#{person.name} is into Stargazing"
end

4.12_code.rb

class Person
  PEOPLE = []
	attr_reader :name, :hobbies, :friends
	
	def initialize(name)
		@name = name
                @hobbies = []
                @friends = []
		PEOPLE << self
	end
	
	def has_hobbies(hobby)
		@hobbies << hobby
	end
	
	def has_friends(friend)
		@friends << friend
	end

4.13_code.rb

  def self.method_missing(m, *args)
		method = m.to_s
		if method.start_with?("all_with_")
			attr = method[9..-1]
			
			if self.public_method_defined?(attr)
				PEOPLE.find_all do |person|
					person.send(attr).include?(args[0])
				end
			else
				raise ArgumentError, "Can't find #{attr}"
			end
		else
			super
		end
	end
end

4.12 line 7 and 8, doing this fixes it:

@Hobbies = []
@friends = []

and this is how it was in the book :-|, I made a mistake while typing...

You can modify the gist to reflect the change.

Btw. I would change the has_XYZ methods to return self. The current implementation leaks the Array member.

rklemme thank you! I edited the gist.

About the has_XYZ methods... and how I should add return self line, could you please explain the Array leaking part? It would help me understand things.