poor orm (ruby+mysql2)

poor_orm.rb

require 'mysql2'

client = Mysql2::Client.new(:host => "localhost", :username => "root", :password => "password", :database => "ar_sample")
results = client.query('DROP TABLE if exists pet')
results = client.query('CREATE TABLE pet (id INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,  name CHAR(30), owner_name CHAR(20), age SMALLINT(6));')

class String
  def to_sql; "\"#{self.to_s}\""; end
end

class Numeric
  def to_sql; self.to_s; end
end

module Model
  module ClassMethods
    attr_reader :connection
    def setup(mysql)
      @connection = mysql

      custom_field_names = connection.query("SHOW COLUMNS FROM #{table_name};").collect{|row| row["Field"] } - ["id"]
      custom_field_names.each do |field_name|         
        define_method(field_name) do 
          instance_variable_get("@#{field_name}")
        end
        define_method("#{field_name}=") do |new_value|
          old_value = instance_variable_get("@#{field_name}") 
          instance_variable_set("@#{field_name}", new_value)
          @changed_attributes << field_name if old_value != new_value && !@changed_attributes.include?(field_name)
        end
      end
    end

    def table_name
      self.name.downcase
    end

    def all
      connection.query("select * from #{table_name}").collect {|row| materialize(row) }
    end

    def find(search_id)
      results = connection.query("select * from #{table_name} where id = #{search_id}").to_a
      results.size > 0 ? materialize(results.first) : nil
    end

    private
    def materialize(hash_data)
      model_instance = self.new
      model_instance.each do |k, v|
       model_instance.instance_variable_set("@#{k}", v)
      end
      model_instance
    end
  end

  class Base
    extend(ClassMethods)
    attr_reader :id

    def initialize()
      @changed_attributes = []
    end

    def new_record?
      @id.nil?
    end

    def save
      return true if @changed_attributes.size == 0
      if (new_record?)
        self.class.connection.query("INSERT INTO #{self.class.table_name} (#{@changed_attributes.sort.join(", ")}) VALUES (#{@changed_attributes.sort.collect{|a| "#{instance_variable_get("@#{a}").to_sql}" }.join(", ")})")
        @id = self.class.connection.last_id
      else
        query = "UPDATE #{self.class.table_name} set #{@changed_attributes.sort.collect{|a| "#{a} = #{instance_variable_get("@#{a}").to_sql}"}.join(", ")} where id = #{@id};"
        r = self.class.connection.query(query)
      end
      @changed_attributes = []
    end
  end
end

class Pet < Model::Base
end

Pet.setup(client)

p = Pet.new
p.name = "Bobik"
p.owner_name = "Dmitry"
p.age = 10
p.save