How to get table of ActiveRecord model instances ids.
>> SomeModel.connection.select_values SomeModel.send(:construct_finder_sql, :select =>:id, :limit => 20)
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
