如果用户已经有东西在购物车里，建议他可以买什么

I am developing e-shop where I will sell food. I want to have a suggestion box where I would suggest what else my user could buy based on what he's already have in cart. If he has beer, I want him to suggest chips and other things by descending precentage of probability that he'll buy it too. But I want that my algorithm would learn to suggest groceries based on the all users' previous purchases. Where should I start? I have groceries table I am developing e-shop where I will sell food.