Easing the use of EmbeddedDocument pattern in Ruby

gistfile1.rb

require 'json'
require 'date'

# Embedders

def scalar
  lambda { |x| x }
end

def date
  lambda { |x| Date.parse(x)  }
end

# Embedder combinators

def sequence_of(e)
  lambda { |array|
    array.map {|x| e.call(x)}
  }
end

def defaulted(e, default)
  lambda { |value|
    value.nil? ? default : e.call(value)
  }
end

# A super-class for embedded document classes. As can be seen, its class is itself an embedder.

class EmbeddedDocument < Struct.new(:data)
  def self.key(name, embedder, accessor_name = nil)
    name = name.to_s
    accessor_name ||= name
    define_method(accessor_name) do
      embedder.call(self.data[name])
    end
  end

  def self.call(value)
    self.new(value)
  end

  def method_missing(sym)
    self.data[sym.to_s]
  end
end

# Use:

class Item < EmbeddedDocument
end

class Delivery < EmbeddedDocument
  key :shipDate, date, :ship_date
  key :items, sequence_of(Item)
end

class Order < EmbeddedDocument
  key :deliveries, sequence_of(Delivery)
  key :items, sequence_of(Item)

  def quantity_for(a_product)
    item = items.detect{|i| a_product == i.product}
    item ? item.quantity : 0
  end
end

order_hash = JSON.parse(
  '{ "id": 1234,
    "customer": "martin",
    "items": [
      {"product": "talisker", "quantity": 500},
      {"product": "macallan", "quantity": 800},
      {"product": "ledaig",   "quantity": 1100}
    ],
    "deliveries": [
      { "id": 7722,
        "shipDate": "2013-04-19",
        "items": [
          {"product": "talisker", "quantity": 300},
          {"product": "ledaig",   "quantity": 500}
        ]
      },
      { "id": 6533,
        "shipDate": "2013-04-18",
        "items": [
          {"product": "talisker", "quantity": 200},
          {"product": "ledaig",   "quantity": 300},
          {"product": "macallan", "quantity": 300}
        ]
      }
    ]
  }'
)

order = Order.new(order_hash)
p order.deliveries
p order.quantity_for('talisker')