Duplicate NSDates in Core Data, but not Duplicate NSStrings with the same code

Question

I have a simple two-tabbed table view controller. The user presses the plus button and is taken modally to another view controller to add text into textfields and select a date from the UIDatePicker.

Everything is working well, except for a duplication problem with Core Data and my dates.

In the app, the first table view displays occasions based on the entry added to the text fields but I have put in some code to check that the occasion doesn't already exist before creating it. What this means is if you have "Wedding" as an occasion and you save, if you enter another entry with "Wedding" as the occasion, rather than creating two cells in the table view with Wedding, it creates just one and when you click on it, it goes another view controller to display all entries for Wedding.

That is working well.

However when it comes to the UIDatePicker and selecting dates, duplicated items are being created.

The model is:

Transaction Entity Occasion Entity Date Entity

The transaction Entity has a relationship to the Occasion and Date Entity.

Let's look at some code:

The save method in the modal view controller:

- (IBAction)save:(id)sender
{
    NSManagedObjectContext *context = [self managedObjectContext];
    Transaction *transaction = [NSEntityDescription insertNewObjectForEntityForName:@"Transaction" inManagedObjectContext:context];
    Occasion *enteredOccasion = (Occasion *)[Occasion occasionWithTitle:self.occasionTextField.text inManagedObjectContext:context];
    transaction.occasion = enteredOccasion;
    // Code to save the date as well - shown below
}

That is calling the occasionWithTitle method which does the NSFetchRequest check:

+ (Occasion *)occasionWithTitle:(NSString *)title inManagedObjectContext:(NSManagedObjectContext *)context
{
    Occasion *occasion = nil;

    // Creating a fetch request to check whether the occasion already exists
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Occasion"];
    request.predicate = [NSPredicate predicateWithFormat:@"title = %@", title];
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"title" ascending:YES];
    request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];

    NSError *error = nil;
    NSArray *occasions = [context executeFetchRequest:request error:&error];
    if (!occasions)
    {
        // Handle Error
    }
    else if (![occasions count])
    {
        // If the occasions count is 0 then let's create it
        occasion = [NSEntityDescription insertNewObjectForEntityForName:@"Occasion" inManagedObjectContext:context];
        occasion.title = title;
    }
    else
    {
        // If the object exists, just return the last object .
        occasion = [occasions lastObject];
    }
    return occasion;
}

The code for the date picker, also in the save method is:

Date *date = (Date *)[Date occasionWithDate:self.datePicker.date inManagedObjectContext:context];
transaction.dates = date;

Which calls:

+ (Date *)occasionWithDate:(NSDate *)enteredDate inManagedObjectContext:(NSManagedObjectContext *)context
{
    Date *date = nil;
    // Creating a fetch request to check whether the date already exists
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Date"];
    request.predicate = [NSPredicate predicateWithFormat:@"dateOfEvent = %@", enteredDate];
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateOfEvent" ascending:YES];
    request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];

    NSError *error = nil;
    NSArray *dates = [context executeFetchRequest:request error:&error];
    if (!dates)
    {
        // Handle Error
    }
    else if (![dates count])
    {
        // If the dates count is 0 then let's create it
        date = [NSEntityDescription insertNewObjectForEntityForName:@"Date" inManagedObjectContext:context];
        date.dateOfEvent = enteredDate;
    }
    else
    {
        // If the object exists, just return the last object .
        date = [dates lastObject];
    }
    return date;
}

To me, the code looks the same but of course one is passing a NSString and one is passing a selected NSDate from a UIDatePicker.

The result is when it comes to the second tabbed table view (for dates), if I create a new Transaction with 2 December 2013 as the date, and then create another entry on 2 December 2013, it creates two separate cells in the table view for the same date, which of course is not right.

Any help on this maddening issue would be very appreciated!

EDIT: On a related note, I am taking the Date selected from the DatePicker and having that displayed as the Section title of the TableView with specific formatting. I am doing that using this code:

-(NSString *)sectionDateFormatter
{
    return [NSDateFormatter localizedStringFromDate:self.dates.dateOfEvent
                                          dateStyle:NSDateFormatterLongStyle
                                          timeStyle:NSDateFormatterNoStyle];
}

And calling this in the sectionNameKeyPath.

Answer 1

Ok, I am offering a different answer because you asked in responding to @Tom whether you should diddle strings to make this work. The answer is emphatically no.

I've done a LOT of date programming in Java and in Objective-C. Java's date stuff (Date/Calendar) was so lame and inadequate, first another team came along (Joda-Time) and now it's being completely redone in 8 (JSR 310). In Java, there used to be easy ways to construct things with days, etc., but apparently, all that code was done outside the purview of the calendar so when the language blew up, it quickly became apparent that it was a mess and didn't work and a huge slew of deprecations resulted.

In Cocoa, however, you have the ability to do all these things by using date components. So you can take one date, then just pull out the components from it that you are interested in, and construct another date with just those components. That's the correct way to program around having values that you are not interested in (e.g. minutes, seconds, milliseconds).

NSDateComponents *components = [[NSDateComponents alloc] init];
[components setDay:6];
[components setMonth:5];
[components setYear:2004];

NSInteger weekday = [components weekday]; 

Even if you are just going to be doing a little date programming, consider watching the WWDC 2013 session. It was surprisingly, not one of the best sessions, but it does cover some things that are a hassle.

But the date Date and Time Programming Guide is must reading, and really outstanding.

Answer 2

Those dates aren't really the same, they're just on the same day. UIDatePicker gives you a full NSDate, which includes the time of day. The time of day seems to be fixed when you load the picker, but if you load the same view more than once, you'll get different times. For example, when messing around with Apple's UICatalog demo, I get 2013-12-03 17:20:11 +0000 when selecting December 3. If I back out via the navigation controller and then reload the same view, I get 2013-12-03 17:20:22 +0000. Those are the same date, as the term is usually used, but they're not the same NSDate. They won't match in a predicate even though they're both on December 3.

If you're only interested in the date, not the time of day, you need to convert the NSDate from the picker into something that only has the date, with either known fixed time values or no time values. A couple of possibilities:

1. Use NSDateFormatter to get a string for the date, using a date format that only includes the year, month, and day.
2. Use NSCalendar to get the year, month, and day components for the NSDate that you get from the picker. Then either just store those values in your model, or convert those back into an NSDate with fixed, known values for the time of day (maybe make them all zero).

The second will be quicker when fetching values (since you'll be filtering on numeric values). The first may be easier to write, but filtering on string values will be slower.

If using the date components approach (recommended) do something like this with the date you get from the picker:

NSDateComponents *components = [[NSCalendar currentCalendar]
    components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit
    fromDate:date];
NSInteger year = components.year;
NSInteger month = components.month;
NSInteger day = components.day;

Then save those three values in your data model instead of an NSDate. Or combine them into a single integer value and save that:

NSInteger dateIndex = year * 10000 + month * 100 + day;

That will give you an integer value of 20131202 for December 2, 2013.

Whichever approach you use, do the same thing when fetching.