my_object.rb

class MyObject < ActiveRecord::Base  
  class JsonValidator < ActiveModel::EachValidator
    def initialize(options)
      options.reverse_merge!(:message => :invalid)
      super(options)
    end

    def validate_each(record, attribute_before_typecast, value)
      attribute = attribute_before_typecast.to_s.sub('_before_type_cast', '')
      value = value.strip if value.is_a?(String)