store.rb

require 'active_support/core_ext/hash/indifferent_access'

module ActiveRecord
  # Store gives you a thin wrapper around serialize for the purpose of storing hashes in a single column.
  # It's like a simple key/value store baked into your record when you don't care about being able to
  # query that store outside the context of a single record.
  #
  # You can then declare accessors to this store that are then accessible just like any other attribute
  # of the model. This is very helpful for easily exposing store keys to a form or elsewhere that's
  # already built around just accessing attributes on the model.
  #
  # Make sure that you declare the database column used for the serialized store as a text, so there's
  # plenty of room.
  #
  # You can set custom coder to encode/decode your serialized attributes to/from different formats.
  # JSON, YAML, Marshal are supported out of the box. Generally it can be any wrapper that provides +load+ and +dump+.
  #
  # Examples:
  #
  #   class User < ActiveRecord::Base
  #     store :settings, accessors: [ :color, :homepage ], coder: JSON
  #   end
  #
  #   u = User.new(color: 'black', homepage: '37signals.com')
  #   u.color                          # Accessor stored attribute
  #   u.settings[:country] = 'Denmark' # Any attribute, even if not specified with an accessor
  #
  #   # There is no difference between strings and symbols for accessing custom attributes
  #   u.settings[:country]  # => 'Denmark'
  #   u.settings['country'] # => 'Denmark'
  #
  #   # Add additional accessors to an existing store through store_accessor
  #   class SuperUser < User
  #     store_accessor :settings, :privileges, :servants
  #   end
  #
  # The stored attribute names can be retrieved using +stored_attributes+.
  #
  #   User.stored_attributes[:settings] # [:color, :homepage]
  #
  # == Overwriting default accessors
  #
  # All stored values are automatically available through accessors on the Active Record
  # object, but sometimes you want to specialize this behavior. This can be done by overwriting
  # the default accessors (using the same name as the attribute) and calling
  # <tt>read_store_attribute(store_attribute_name, attr_name)</tt> and
  # <tt>write_store_attribute(store_attribute_name, attr_name, value)</tt> to actually
  # change things.
  #
  #   class Song < ActiveRecord::Base
  #     # Uses a stored integer to hold the volume adjustment of the song
  #     store :settings, accessors: [:volume_adjustment]
  #
  #     def volume_adjustment=(decibels)
  #       write_store_attribute(:settings, :volume_adjustment, decibels.to_i)
  #     end
  #
  #     def volume_adjustment
  #       read_store_attribute(:settings, :volume_adjustment).to_i
  #     end
  #   end
  module Store
    extend ActiveSupport::Concern

    included do
      class_attribute :stored_attributes, instance_accessor: false
      self.stored_attributes = {}
    end

    module ClassMethods
      def store(store_attribute, options = {})
        serialize store_attribute, IndifferentCoder.new(options[:coder])
        store_accessor(store_attribute, options[:accessors]) if options.has_key? :accessors
      end

      def store_accessor(store_attribute, *keys)
        keys = keys.flatten
        keys.each do |key|
          define_method("#{key}=") do |value|
            write_store_attribute(store_attribute, key, value)
          end

          define_method(key) do
            read_store_attribute(store_attribute, key)
          end
        end

        self.stored_attributes[store_attribute] ||= []
        self.stored_attributes[store_attribute] |= keys
      end
    end

    protected
      def read_store_attribute(store_attribute, key)
        attribute = initialize_store_attribute(store_attribute)
        attribute[key]
      end

      def write_store_attribute(store_attribute, key, value)
        attribute = initialize_store_attribute(store_attribute)
        if value != attribute[key]
          send :"#{store_attribute}_will_change!"
          attribute[key] = value
        end
      end

    private
      def initialize_store_attribute(store_attribute)
        attribute = send(store_attribute)
        unless attribute.is_a?(HashWithIndifferentAccess)
          attribute = IndifferentCoder.as_indifferent_hash(attribute)
          send :"#{store_attribute}=", attribute
        end
        attribute
      end

    class IndifferentCoder # :nodoc:
      def initialize(coder_or_class_name)
        @coder =
          if coder_or_class_name.respond_to?(:load) && coder_or_class_name.respond_to?(:dump)
            coder_or_class_name
          else
            ActiveRecord::Coders::YAMLColumn.new(coder_or_class_name || Object)
          end
      end

      def dump(obj)
        @coder.dump self.class.as_indifferent_hash(obj)
      end

      def load(yaml)
        self.class.as_indifferent_hash @coder.load(yaml)
      end

      def self.as_indifferent_hash(obj)
        case obj
        when HashWithIndifferentAccess
          obj
        when Hash
          obj.with_indifferent_access
        else
          HashWithIndifferentAccess.new
        end
      end
    end
  end
end