Question

I have a simple data model (Core Data), but here is a stripped down version of it:

Account
----------------------------------------------------
NSSet<Transaction> transactions

Transaction
----------------------------------------------------
Account  account
NSNumber amount
NSNumber type ( type ∈ ( 0->'Credit', 1->'Debit' ) )

I want to get an account’s total balance, however, I don’t want to mark a debit as a negative number. I just want it to be an amount with a type of debit.

Currently, before I added the debits in, I could get the balance via this function in Account:

- (NSNumber *)currentBalance
{
    return [self.transactions valueForKeyPath:@"@sum.amount"];
}

Obviously, that will only work if I make debits negative. Is there an elegant solution here, to perhaps filter the set into credits and debits, sum those, and perform a difference? Will that maintain accuracy? I’ve looked into NSPredicate, but I’m not sure how exactly to proceed.

Answer 1

Here is what my research has led me to:

- (NSNumber *)currentBalance
{
    // credits
    NSPredicate *creditPredicate = [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
        return [((Transaction *)evaluatedObject).type intValue] == kTransactionTypeCredit;
    }];

    NSSet *credits = [self.transactions filteredSetUsingPredicate:creditPredicate];

    NSDecimalNumber *totalCredits = [credits valueForKeyPath:@"@sum.amount"];

    // debits
    NSPredicate *debitPredicate = [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
        return [((Transaction *)evaluatedObject).type intValue] == kTransactionTypeDebit;
    }];

    NSSet *debits = [self.transactions filteredSetUsingPredicate:debitPredicate];

    NSDecimalNumber *totalDebits = [debits valueForKeyPath:@"@sum.amount"];

    // do the math
    return [totalCredits decimalNumberBySubtracting:totalDebits];
}

Whilst it works, it seems like an excessive amount of work. Is it the best way?