Simple model:
Products, which have weights (can be mixed - ounces, grams, kilograms etc)
Cagtegories, which have many products in them
REST endpoints:
/products - get all products and post to add a new one
/products/id - delete,update,patch a single product
/categories/id - delete,update,patch a single category
/categories - get all categories and post to add a new one
The question is that the frontend client wants to display a chart of the total weight of products in ALL categories. Imagine a bar chart or a Pie chart showing all categories, and the total weight of products in each.
Obviously the product model has a 'weight_value' and a 'weight_unit' so you know the weight of a product and its measure (oz, grams etc).
I see 2 ways of solving this:
In the category model, add a computed field that totals the weight of all the products in a category. The total is calculated on the fly for that category (not stored) and so is always up to date. Note the client always needs all categories (eg. to populate drop downs when creating a product, you have to choose the category it belongs to) so it now will automatically always have the total weight of each category. So constructing the chart is easy - no need to get the data for the chart from the server, it's already on the client. But first time loading that data might be slow. Only when a product is added will the totals be stale (insignificant change though to the overall number and anyway stale totals are fine).
Create a separate endpoint, say categories/totals, that returns for all categories: a category id, name and total weight. This endpoint would loop through all the categories, calculating the weight for each and returning a category dataset with weights for each.
The problem I see with (1) is that it is not that performant. I know it's not very scalable, especially when a category ends up with a lot of products (millions!) but this is a hobby project so not worried about that.
The advantage of (1) is that you always have the data on the client and don't have to make a separate request to get the data for the chart.
However, the advantage of (2) is that every request to category/id does not incur a potentially expensive totalling operation (because now it is inside its own dedicated endpoint). Of course that endpoint will have to do quite a complex database query to calculate the weights of all products for all categories (although handing that off to the database should be the way as that is what db's are good for!)
I'm really stumped on which is the better way to do this. Which way is more true to the RESTful way (HATEOS basically)?
I would go with 2. favouring scalability and best practices. It does not really make any sense to perform the weight calculations every time the category is requested and even though you may not anticipate this to be a problem since it is a 'hobby' project, it's always best to avoid shortcuts where the benefits are minimal ( or so experience has taught me !).
Choosing 1, the only advantage would be that you have to set up one less endpoint and one extra call to get the weight total - the extra call shouldn't add too much overhead, and setting up the extra endpoint shouldn't take up too much effort.
Regardless of whether you choose 1 or 2, I would consider caching the weight total ( for a reasonable amount of time, depending on accuracy required ) to increase performance even further.