AREL merge (&) causes redundant joins

[nikhilgupte]

Without meta_where:

> (UserVote.joins(:user) & User.verified).to_sql
=> "SELECT `votes`.* FROM `votes` INNER JOIN `users` ON `users`.`id` = `votes`.`user_id` WHERE `votes`.`type` = 'UserVote' AND (verified_at is not null)"

With meta_where:

> (UserVote.joins(:user) & User.verified).to_sql
=> "SELECT `votes`.* FROM `votes` INNER JOIN `users` ON `users`.`id` = `votes`.`user_id` INNER JOIN `users` `voting_users_votes` ON `voting_users_votes`.`id` = `votes`.`user_id` WHERE `votes`.`type` = 'UserVote' AND (verified_at is not null)"