The generated SQL is semantically correct. Rails wraps each level of scope in its own set of parenthesis in order to ensure correct execution order.
There is no difference between the following when executed:
SELECT `user_sessions`.* FROM `user_sessions` WHERE `user_sessions`.`key` = '123345' AND (accessed_at >= '2013-10-12 10:50:52') LIMIT 1
and
SELECT `user_sessions`.* FROM `user_sessions` WHERE `user_sessions`.`key` = '123345' AND accessed_at >= '2013-10-12 10:50:52' LIMIT 1
Are you sure that there is a record that satisfies that query?