How to build subsections in an NSFetchedResultsController

I'm building an expense tracker where an Expense can belong to only one Category but can have multiple Tags. This is my object graph:

In the screen where I list all the expenses in a table view, I want the expenses to be grouped by date (the sectionDate), and then by Category (or, using a segmented control, by Tag). This is the intended UI:

I can already make an NSFetchedResultsController query all expenses, section them by date, then by category, but I can't get the (1) total for the category and (2) the list of expenses in it. How might I proceed to do that? This is my current code:

let fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult> = {
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Expense")
    fetchRequest.resultType = .dictionaryResultType

    fetchRequest.sortDescriptors = [
        NSSortDescriptor(key: #keyPath(Expense.sectionDate), ascending: false)
    ]

    fetchRequest.propertiesToFetch = [
        #keyPath(Expense.sectionDate),
        #keyPath(Expense.category)
    ]
    fetchRequest.propertiesToGroupBy = [
        #keyPath(Expense.sectionDate),
        #keyPath(Expense.category)
    ]

    let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest,
                                                              managedObjectContext: Global.coreDataStack.viewContext,
                                                              sectionNameKeyPath: #keyPath(Expense.sectionDate),
                                                              cacheName: nil)
    return fetchedResultsController
}()