validator.rb

#!/usr/bin/env ruby

require 'active_record'

ActiveRecord::Base.establish_connection(
  :adapter  => 'sqlite3',
  :database => ':memory:'
)

ActiveRecord::Schema.define do
  create_table :documents, :force => true do |t|
  end
end

module HstoreValidation
  def validates_hstore(field, &block)
    validation_class = Class.new do
      include ActiveModel::Validations

      def self.name
        '(validations)'
      end

      def initialize(data)
        @data = data
      end

      def read_attribute_for_validation(attr_name)
        @data[attr_name]
      end
    end
    validation_class.class_eval &block

    validate do
      validator = validation_class.new(self[field])

      if validator.invalid?
        validator.errors.each do |attr, text|
          self.errors.add(attr, text)
        end
      end
    end
  end
end

class Document < ActiveRecord::Base
  extend HstoreValidation

  validates_hstore :data do
    validates_length_of :title, :maximum => 50
  end
end

d = Document.new
d[:data] = {title: 'abc' * 51}

p d.valid?
p d.errors

d[:data][:title] = 'shorter title'

p d.valid?
p d.errors