solved.rb

irb(main)> user = User.first
  User Load (2.9ms)  SELECT  "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT ?  [["LIMIT", 1]]
=> #<User id: 1, created_at: "2017-07-13 18:28:46", updated_at: "2017-07-13 18:28:46">
irb(main)> user.reviews.first.author.object_id == user.object_id
  CarReview Load (1.4ms)  SELECT  "reviews".* FROM "reviews" WHERE "reviews"."author_id" = ? ORDER BY "reviews"."id" ASC LIMIT ?  [["author_id", 1], ["LIMIT", 1]]
=> true
irb(main)> user.cars.first.user.object_id == user.object_id
  Car Load (1.7ms)  SELECT  "cars".* FROM "cars" WHERE "cars"."user_id" = ? ORDER BY "cars"."id" ASC LIMIT ?  [["user_id", 1], ["LIMIT", 1]]
=> true