has_custom_fields_backed_by_json.rb

# inspired by http://faxon.org/2015/02/03/edit-rails-activerecord-json-attributes-in-html-forms
# NOTE requires `custom_fields` column!
#
# class Foo
#   include HasCustomFieldsBackedByJSON
#   custom_fields_keys :bar, :baz
# end

module HasCustomFieldsBackedByJSON
  extend ActiveSupport::Concern

  def custom_fields_attributes=(attrs)
    write_attribute(
      :custom_fields,
      read_attribute(:custom_fields).merge(attrs)
    )
  end

  def custom_fields
    CustomFieldset.new(
      read_attribute(:custom_fields), self.class.custom_fields_keys
    )
  end

  class_methods do
    def custom_fields_keys(*keys)
      # kind of weird this is both a reader and writer, hmm
      if keys.any?
        instance_variable_set("@custom_fields_keys", keys)
        delegate *keys, to: :custom_fields, allow_nil: true
      else
        instance_variable_get("@custom_fields_keys") || []
      end
    end
  end

  class CustomFieldset
    attr_reader :fields_keys

    def initialize(hash, fields_keys)
      @fields_keys = fields_keys
      fields_keys.each do |key|
        singleton_class.class_eval do; attr_reader key; end
        instance_variable_set("@#{key}", hash[key.to_s])
      end
    end

    def persisted?() false; end
    def new_record?() false; end
    def marked_for_destruction?() false; end
    def _destroy() false; end
  end
end