Modeling exerciseThe grocery store’s checkout model process approach
time to read 2 min | 372 words

I posted about the grocery store checkout process exercise before. Now I want to see if I can do a short outline on how I would handle this.

The key aspect from my perspective is that we need to separate the notion of the data we have and the processing of the data. That means that we are going to have the following model:

public class ShoppingCart
{
   public List<ProductInShoppingCart> Products {get;set;}
   public List<Discount> Discounts { get;set; }
}

public class ProductInShoppingCart
{
   public string ProductId;
   public Discount Discount;
}

Note that we explicitly do not have a quantity field here. If we purchase 6 bottles of milk, that would appear three times in the cart. Why is that?

Let us assume that we have a sale for 2 bottles of milk for 20% discount or a 3 +1 bottles of milk offer. Consider the kind of code you would have to write in the offer code:

  • Find all products that have this offer and have 4 items without discount.
  • Add the discount to those products.
  • After searching for products without discount, need to search for products with a discount, but that we can apply this to and get a better option.

In this case, we start by doing:

  • Add bottle of milk
  • Add bottle of milk – 2 for 20% discount is triggered.
  • Add bottle of milk
  • Add bottle of milk – 3+1 offer is triggered, removing the previous discount.

Because this is likely going to be complex, I’m going to be writing this once. A set of offers and the kind of rules that we want. Then we will give the users the ability to define those rules.

Note that we keep the raw data (products) and the transformations (discounts) separate, so we can always reapply everything without losing any data.

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More posts in "Modeling exercise" series:

  1. (28 Oct 2014) The grocery store’s checkout model process approach
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