parent = { 'parent' => nil, 'id' => 3 }
child = { 'parent' => parent, 'id' => 7 }

{
  "parent": { "parent": null, "id": 3, ...etc.},
  "id": 7,
  ...etc.
}

class Item
  def initialize(data)
    @data = data
  end

  def parent
    @parent ||= Item.new(data['parent']) if data['parent']
  end

  def id
    data['id']
  end

  private

  attr_reader :data
end

describe 'children' do
  let(:subject1) { Item.new(child) }

  it 'is an item' do
    subject1.class.must_equal(Item)
  end

  it 'has an id' do
    subject1.id.must_equal(7)
  end

  it 'has a parent' do
    subject1.parent.id.must_equal(3)
  end
end

describe 'parent' do
  let(:subject2) { subject1.parent }

  it 'is also an item' do
    subject2.class.must_equal(Item)
  end

  it 'does not have a parent' do
    assert_nil(subject2.parent)
  end
end

module Handlers
  module JSONHandler
    require 'json'

    def from_json json
      hash = JSON.parse(json)
      recursive hash
    end

    # You might want to add an extra check to stop an infinite loop
    # Beware the data!
    def recursive hash
      return nil if hash.nil?
      return hash unless hash.respond_to? :fetch
      Item.new id: hash["id"], parent: recursive(hash["parent"])
    end
  end
end


class Item
  extend Handlers::JSONHandler

  def self.items
    @items ||= {}
  end


  def initialize(id:, parent:nil)
    @parent = parent
    @id = id
    self.class.items[@id] = self
  end

  attr_reader :parent, :id

  def to_h
    h = {id: @id, parent: (@parent && parent.id) }
    h.reject{|k,v| v.nil? }
  end

  alias_method :to_hash,:to_h
end
# => Item
parent = Item.new id: 3
# => #<Item:0x007fa94a4010c8 @parent=nil, @id=3>
child = Item.new id: 7, parent: parent
# => #<Item:0x007fa94a9abe38 @parent=#<Item:0x007fa94a4010c8 @parent=nil, @id=3>, @id=7>
Item.items
# => {3=>#<Item:0x007fa94a4010c8 @parent=nil, @id=3>, 7=>#<Item:0x007fa94a9abe38 @parent=#<Item:0x007fa94a4010c8 @parent=nil, @id=3>, @id=7>}
Item.items[3]
# => #<Item:0x007fa94a4010c8 @parent=nil, @id=3>
Item.items[7]
# => #<Item:0x007fa94a9abe38 @parent=#<Item:0x007fa94a4010c8 @parent=nil, @id=3>, @id=7>
Item.items[7].parent
# => #<Item:0x007fa94a4010c8 @parent=nil, @id=3>
parent.to_h
# => {:id=>3}
child.to_hash
# => {:id=>7, :parent=>3}
json_parent = parent.to_h.to_json
# => "{"id":3}"
json_child = child.to_h.to_json
# => "{"id":7,"parent":3}"
parent2 = Item.from_json json_parent
# => #<Item:0x007fb543e26940 @parent=nil, @id=3>
child2 = Item.from_json json_child
# => #<Item:0x007fb5431d7270 @parent=3, @id=7>

def self.Item(*args, **keywords)
  case args.first
  when String # it's JSON
    Item.from_json args.first
  when Hash
    # you might want to nick the recursive bit from the handler
    # or write an Hash handler etc
    Item.new id: args.first["id"], parent: args.first["parent"]
  when Item
    args.first
  else
    Item.new id: keywords[:id], parent: keywords[:parent]
  end
end

Item(parent)
# => #<Item:0x007fb9b2765160 @parent=nil, @id=3>
Item(child)
# => #<Item:0x007fb9b272f330 @parent=3, @id=7>
Item(json_parent)
# => #<Item:0x007fb9b26fdd30 @parent=nil, @id=3>
Item(json_child)
# => #<Item:0x007fb9b26c7dc0 @parent=3, @id=7>
Item(parent.to_h)
# => #<Item:0x007fb9b26965b8 @parent=nil, @id=3>
Item(child.to_h)
# => #<Item:0x007fb9b2664928 @parent=3, @id=7>
Item(id: 3)
# => #<Item:0x007fb9b262ed50 @parent=nil, @id=3>
Item(id: 7, parent: parent)
# => #<Item:0x007fb9b25fc990 @parent=#<Item:0x007fb9b23bd7b0 @parent=nil, @id=3>, @id=7>