[#<User
  id: 5,
  name: "Dave",
  email: "example@gmail.com",
  info:
   {"years_old"=>"30",
    "recent_purchases"=>["car", "house", "boat"]}>]

Let's assume that I'm saving recent purchases to {info: { recent_purchases: []) in user model

I want to make a scope to get users with only 'car' in recent purchases. I can't change database structure so I have to figure it out

I've tried this User.where("info->>'recent_purchases' IN (?)", 'car') but it doesn't work
This also didn't work - User.where("info->>'recent_purchases' = ?", ["car"])

Edit: Thanks to @Jacob, this worked perfectly!

User.where("where info -> 'recent_purchases' ? :car and jsonb_array_length(info -> 'recent_purchases') = 1", car: 'car')

Edit2:
If you want to create a scope for a user that has a car in his recent_purchases, that's how you do it:

scope :where_cars_included, -> { where("info::jsonb -> 'recent_purchases' ? :purchase", purchase: 'car') }

For scope with ONLY cars in recent_purchases:

scope :only_cars, -> { where("info::jsonb -> 'recent_purchases' ? :purchase and json_array_length(info -> 'recent_purchases') = 1", purchase: 'car') }

There is a small guide that can help:

How to query PostgreSQL's json fields from Rails