arel in rails 2.3

arel_back.rb

# put into lib/arel_back.rb
module ArelBack

  module ActiveRecord
    def self.included(base)
      base.extend(ClassMethods)
    end

    module ClassMethods

      def arel_engine
        return @arel_engine if @arel_engine
        adapter = connection.adapter_name.to_sym
        @arel_engine = ArelBack::Engine.new(connection, Arel::Visitors.const_get(adapter))
      end

      def arel
        Arel::Table.new(table_name, arel_engine)
      end

    end

  end


  class Connection
    delegate :primary_key, :table_exists?, :columns, :quote_table_name, :quote_column_name, :quote, :to => :@raw_connection

    attr_reader :visitor

    def initialize(raw_connection, visitor_klass)
      @raw_connection = raw_connection
      @visitor = visitor_klass.new(self)
    end

    def schema_cache
      ArelBack::SchemaCache.new(self)
    end
  end



  class Engine
    attr_reader :connection

    def initialize(raw_connection, visitor_klass)
      @raw_connection = raw_connection
      @connection = ArelBack::Connection.new(@raw_connection, visitor_klass)
    end
  end


  class SchemaCache
    attr_reader :columns, :columns_hash, :primary_keys, :tables
    attr_reader :connection

    def initialize(conn)
      @connection = conn
      @tables     = {}

      @columns = Hash.new do |h, table_name|
        h[table_name] = conn.columns(table_name, "#{table_name} Columns")
      end

      @columns_hash = Hash.new do |h, table_name|
        h[table_name] = Hash[columns[table_name].map { |col|
          [col.name, col]
        }]
      end

      @primary_keys = Hash.new do |h, table_name|
        h[table_name] = table_exists?(table_name) ? conn.primary_key(table_name) : nil
      end
    end

    # A cached lookup for table existence.
    def table_exists?(name)
      return @tables[name] if @tables.key? name

      @tables[name] = connection.table_exists?(name)
    end

    # Clears out internal caches
    def clear!
      @columns.clear
      @columns_hash.clear
      @primary_keys.clear
      @tables.clear
    end

    # Clear out internal caches for table with +table_name+.
    def clear_table_cache!(table_name)
      @columns.delete table_name
      @columns_hash.delete table_name
      @primary_keys.delete table_name
      @tables.delete table_name
    end
  end
end

init.rb

  if Rails.version < "3"
    unless ActiveRecord::Base.included_modules.include? ArelBack::ActiveRecord
      ActiveRecord::Base.send(:include, ArelBack::ActiveRecord)
    end
  end