解析：selectKeys方法不起作用

Question

我只想从Parse中检索某些列。但它会下载整个对象：

            let usersQuery = PFQuery(className: "_User")
            usersQuery.whereKey("userId", containedIn: self.memberIds!) // Array of Strings containing the userIds
            usersQuery.selectKeys(["email"])
            usersQuery.findObjectsInBackgroundWithBlock({ (objects: [PFObject]?, error: NSError?) -> Void in
                if error != nil {
                    print(error)
                } else if let users = objects as? [PFUser] {

                    print("objects: \(users)")

                    // prints whole object, not only "email" field
                }
            })

输出：

objects: [<PFUser: 0x7fb1095a7270, objectId: 9Ld9vRPoLZ, localId: (null)> {
email = "iphone5s@mail.com";
fullname = "iPhone 5S";
// ... other fields

}]