Predicate and expression to fetch complex request core data - iphone

Question

I have to make a complex core data fetch request but I don't know if it can be made.
This is my scenario: just one entity (Expense) with these attributes:
Cost (NSDecimalNumber)
Deposit (NSDecimalNumber)
Category (NSString)
Paid (Boolean Value)
The request should return the 3 most expensive categories but these are the rules that must be respected:
If Paid == YES, Expense cost should be added to Expense category total
If Paid == NO && Deposit > 0, Expense deposit should be added to Expense category total
If Paid == NO, nothing should be added to Expense category total
Using NSExpression, I'm able to calculate every total per category but it also includes cost of Expenses not paid.
Is there a way to accomplish this?
Thank you so much!

Answer 1

You could, for example, use a NSFetchRequest:
// Build the fetch request
NSString *entityName = NSStringFromClass([Expense class]);
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = entity;
which filters only relevant expenses:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"(paid == YES) OR ((paid == NO) AND (deposit > 0))"];
request.predicate = predicate;
and sums up the cost and depost attributes:
NSExpressionDescription *(^makeExpressionDescription)(NSString *, NSString *) = ^(NSString *keyPath, NSString *name)
{
// Create an expression for the key path.
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:keyPath];
// Create an expression to represent the function you want to apply
NSExpression *totalExpression = [NSExpression expressionForFunction: #"sum:" arguments: #[keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
// The name is the key that will be used in the dictionary for the return value
expressionDescription.name = name;
expressionDescription.expression = totalExpression;
expressionDescription.expressionResultType = NSDecimalAttributeType;
return expressionDescription;
};
NSExpressionDescription *totalCostDescription = makeExpressionDescription(#"cost", #"totalCost");
NSExpressionDescription *totalDepositDescription = makeExpressionDescription(#"deposit", #"totalDeposit");
// Specify that the request should return dictionaries.
request.resultType = NSDictionaryResultType;
request.propertiesToFetch = #[categoryDescription,
paidDescription,
totalCostDescription,
totalDepositDescription];
and group the results by category and paid status:
// Get 'category' and 'paid' attribute descriptions
NSEntityDescription *entity = [NSEntityDescription entityForName:entityName
inManagedObjectContext:context];
NSDictionary *attributes = [entity attributesByName];
NSAttributeDescription *categoryDescription = attributes[#"category"];
NSAttributeDescription *paidDescription = attributes[#"paid"];
// Group by 'category' and 'paid' attributes
request.propertiesToGroupBy = #[categoryDescription, paidDescription];
You'll get paid and unpaid expenses summed up
NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];
all you need to do is combine (and sort) then:
if (results) {
NSMutableDictionary *combined = [NSMutableDictionary dictionary];
for (NSDictionary *result in results) {
NSString *category = result[#"category"];
BOOL paid = [result[#"paid"] boolValue];
NSDecimalNumber *total = result[paid ? #"totalCost" : #"totalDeposit"];
NSDecimalNumber *sum = combined[category];
if (sum) {
total = [total decimalNumberByAdding:sum];
}
combined[category] = total;
}
NSArray *sortedCategories = [combined keysSortedByValueUsingSelector:#selector(compare:)];
[sortedCategories enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
NSLog(#"Category %#: %#", obj, combined[obj]);
}];
}
else {
NSLog(#"Error: %#", error);
}