In Core Data, how sort an NSFetchRequest depending on the sum of an attribute of a child entity? (SwiftUI) - core-data

Question

I am building an iOS app in SwiftUI for which I have a Core Data model with two entities:
CategoryEntity with attribute: name (String)
ExpenseEntity with attributes: name (String) and amount (Double)
There is a To-many relationship between CategoryEntity and ExpenseEntity (A category can have many expenses).
I’m fetching the categories and showing them in a list together with the sum of the expenses for each category as follows: Link to app screenshot
I would like to add a sort to the fetch request so the categories appear in order depending on the total amount of their expenses. In the example of the previous picture, the order of appearance that I would like to get would be: Tech, Clothes, Food and Transport. I don’t know how to approach this problem. Any suggestions?
In my current implementation of the request, the sorted is done alphabetically:
// My current implementation for fetching the categories
func fetchCategories() {
let request = NSFetchRequest<CategoryEntity>(entityName: "CategoryEntity")
let sort = NSSortDescriptor(keyPath: \CategoryEntity.name, ascending: true)
request.sortDescriptors = [sort]
do {
fetchedCategories = try manager.context.fetch(request)
} catch let error {
print("Error fetching. \(error.localizedDescription)")
}
}

Answer 1

You don't have to make another FetchRequest, you can just sort in a computed property like this:
(I assume your fetched results come into a var called fetchedCategories.)
var sortedCategories: [CategoryEntity] {
return fetchedCategories.sorted(by: { cat1, cat2 in
cat1.expensesArray.reduce(0, { $0 + $1.amount }) >
cat2.expensesArray.reduce(0, { $0 + $1.amount })
})
}
So this sorts the fetchedCategories array by a comparing rule, that looks at the sum of all cat1.expenses and compares it with the sum of cat2.expenses. The >says we want the large sums first.
You put the computed var directly in the View where you use it!
And where you used fetchedCategories before in your view (e.g. a ForEach), you now use sortedCategories.
This will update in the same way as the fetched results do.

Answer 2

One approach would be to include a derived attribute in your CategoryEntity model description which keeps the totals for you. For example, to sum the relevant values from the amount column within an expenses relation:
That attribute should be updated whenever you save your managed object context. You'll then be able to sort it just as you would any other attribute, without the performance cost of calculating the expense sum for each category whenever you sort.
Note that this option only really works if you don't have to do any filtering on expenses; for example, if you're looking at sorting based on expenses just in 2022, but your core data store also has seconds in 2021, the derived attribute might not give you the sort order you want.