union doesn't work for union of fields

[Ana06]

I had expected that the following code:

BsRequest.where(id: BsRequestAction.where(id: [1,2]).select(:bs_request_id).union(Review.where(id: [1,2,3]).select(:bs_request_id)))

to produce the following query:

SELECT  `bs_requests`.* FROM `bs_requests` WHERE `bs_requests`.`id` IN ((SELECT `bs_request_actions`.`bs_request_id` FROM `bs_request_actions` WHERE `bs_request_actions`.`id` IN (1, 2)) UNION (SELECT `reviews`.`bs_request_id` FROM `reviews` WHERE `reviews`.`id` IN (1, 2, 3)))

Instead, it produces:

SELECT  `bs_requests`.* FROM `bs_requests` WHERE `bs_requests`.`id` IN (SELECT `bs_request_actions`.`id` FROM ( (SELECT `bs_request_actions`.`bs_request_id` FROM `bs_request_actions` WHERE `bs_request_actions`.`id` IN (1, 2)) UNION (SELECT `reviews`.`bs_request_id` FROM `reviews` WHERE `reviews`.`id` IN (1, 2, 3)) ) `bs_request_actions`)