Find certain object in Ruby

Question

Always working with Rails but now that I am coding completely without framework I am missing a lot of methods that I am not able to write myself. So I need a little hint here.

So I've got this code

class Foo
   initialize(k)
    (1..k).each do |b|
       Boo.new(b)
    end
end

class Boo
  initialize(a)
    @a = a
  end
end

Foo.new(10)
#and now I want to access one of the Boo's with a certain a (in Rails Boo.find_by_a(2) e.g.)

How do I write a method that uses a certain value "a" to get the specific Boo back with @a = a?

Update:

Okay I added this methot to Boo

  def self.all
    ObjectSpace.each_object(self).to_a
  end

And now I get the right object by:

boo = Boo.all.select{ |b| b.a == 2 }.first

But if I want this to be a method, where would i put it?

Answer 1

DO NOT RELY ON ObjectSpace for this functionality when the GC collects you will lose these objects because Ruby sees them as unneeded.

This is the general concept of what you are intending to do

class Foo 
  attr_reader :boos
  def initialize(k)
    @boos = (1..k).map {|b| Boo.new(b) } 
  end
  def find_boo_with(x)
    @boos.find {|b| b.a == x }
  end
end

class Boo
  attr_reader :a
  def initialize(a)
    @a = a
  end
end

Then you can do this

f = Foo.new(10)
f.boos
#=> [#<Boo:0x256f428 @a=1>,
     #<Boo:0x256f3b0 @a=2>,
     #<Boo:0x256f368 @a=3>,
     #<Boo:0x256f338 @a=4>,
     #<Boo:0x256f2f0 @a=5>,
     #<Boo:0x256f2d8 @a=6>,
     #<Boo:0x256f2a8 @a=7>,
     #<Boo:0x256f290 @a=8>,
     #<Boo:0x256f278 @a=9>,
     #<Boo:0x256f260 @a=10>]
f.find_boo_with(9)
#=> #<Boo:0x256f278 @a=9>

I hope this helps but you really should look into something like RubyMonk, or TryRuby to get a better understanding of how PORO (Plain Old Ruby Objects) work.

You could also do it this way if you really wanted your find methodology just know that no Boo in this case will ever be collected as they will all be stored in a class instance variable called @all_boos which could casue memory issues depending on what a Boo is and how many of them you are creating.

class Boo
   class << self
      def all_boos
        @all_boos ||= []
      end
      def find(x)
        @all_boos.find {|boo| boo.a == x }
      end
      alias_method :all, :all_boos
   end
   attr_reader :a
   def initialize(a)
     @a = a
     self.class.all_boos << self
   end
end

Then you can do

10.times {|i| Boo.new(i) }
Boo.all
#=> [#<Boo:0x256f428 @a=1>,
     #<Boo:0x256f3b0 @a=2>,
     #<Boo:0x256f368 @a=3>,
     #<Boo:0x256f338 @a=4>,
     #<Boo:0x256f2f0 @a=5>,
     #<Boo:0x256f2d8 @a=6>,
     #<Boo:0x256f2a8 @a=7>,
     #<Boo:0x256f290 @a=8>,
     #<Boo:0x256f278 @a=9>,
     #<Boo:0x256f260 @a=10>] 
Boo.find(9)
#=>  #<Boo:0x256f278 @a=9>

Answer 2

If I did understand what you need then I think that you can use ObjectSpace to achieve this. This is the sample code that illustrates the use:

class Foo
  def initialize(k)
    (1..k).each do |b|
       Boo.new(b)
    end
  end
end

class Boo
  attr_reader :a
  def initialize(a)
    @a = a
  end

 def self.find(value)
   ObjectSpace.each_object(Boo).select { |b| b.a == value }.first
 end
end
Foo.new(10)
Boo.find(2).a

Let me know if this is the solution you were looking for.

Answer 3

Not sure I 100% understand your question but using attr_reader in Boo will allow you to access a as if it were a function.

class Foo
  def initialize(k)
    (1..k).each do |b|
       Boo.new(b)
    end
  end
end

class Boo
  attr_reader :a

  def initialize(a)
    @a = a
  end
end

Foo.new(10)

As for the other part of your question, here is a good article explaining how dynamic finders work