active_record_ext.rb

module ActiveRecord
  ##idea is from lovdbyless
  ##These extensions give database specific implementations for common functions
  module ConnectionAdaptersExt
    module MySQL
      def rand
        'RANDOM()'
      end
      def now
        'NOW()'
      end
      def birthyear(fld)
        "YEAR(#{fld})"
      end
      def age(fld)
        #"DATE_FORMAT(FROM_DAYS(TO_DAYS(NOW())-TO_DAYS(#{fld})), '%Y')+0"
        #"DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(#{fld}, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(#{fld}, '00-%m-%d'))"
        "YEAR(CURDATE()) - YEAR(#{fld}) - IF(RIGHT(CURDATE(),5) < RIGHT(#{fld},5),1,0)"
      end
      def least(fld,value,cast=true)
        "least(#{fld},#{value})"
      end
      def boolean_value(val)
        val
      end
    end
    module Posgres
      def rand
        'RAND()'
      end
      def now
        "NOW()"
      end
      def birthyear(fld)
        "date_part('year',#{fld})"
      end
      #age(a) will say the age from today - exactly what we want
      def age(fld)
        "date_part('year', age(#{fld}))"
      end
      def least(fld,value,cast=true)
        if cast
          "least(cast(#{fld} as INTEGER),#{value})"
        else
          "least(#{fld},#{value})"
        end
      end
      def boolean_value(val)
        val
      end
    end
    module SQLite
      def rand
        'RAND()'
      end
      def age(fld)
        "(#{now}-#{fld})"
      end 
      def birthyear(fld)
        "cast(#{fld} as integer)"
      end
      def now
        "datetime('now')"
      end
      def least(fld,value,cast=true)
        "min(#{fld},#{value})"
      end
      def boolean_value(val)
        val ? 't' : 'f'
      end
      def import_csv(file,table)
        ".import #{file} #{table}"
      end
    end
  end
end

case ActiveRecord::Base.connection.adapter_name
when 'MySQL'
  ActiveRecord::ConnectionAdapters::MysqlAdapter.send(:include, ActiveRecord::ConnectionAdaptersExt::MySQL)
when /SQLite/
  ActiveRecord::ConnectionAdapters::AbstractAdapter.send(:include, ActiveRecord::ConnectionAdaptersExt::SQLite)
else # 'PostgreSQL' ...
  ActiveRecord::ConnectionAdapters::AbstractAdapter.send(:include, ActiveRecord::ConnectionAdaptersExt::Posgres)
end

active_record_ext_usage.rb

Account.scoped(:conditions => ['active = ?',ActiveRecord::Base.connection.boolean_value(true)])