Search CoreData for Matching NSDate - Swift - ios

Question

I want to search through all existing objects to see if there are any matching date objects in CoreData:
Dates are currently saved in my CoreData via the start_date attribute with the following format: 2013-08-29 14:27:47 +0000.
I am then letting a user select a date from a UIDatePicker and assigning .date() to variable date.
e.g. My Selected date = 2013-08-29 17:34:23 +0000.
Below is how i search CoreData, using a predicate.
let predicate = NSPredicate(format: "start_date contains[search] %#", date)
let request:NSFetchRequest = NSFetchRequest(entityName: "Project")
let sortDescriptor:NSSortDescriptor = NSSortDescriptor(key: "number", ascending: true)
let sortDescriptorsAry:NSArray = [sortDescriptor]
request.sortDescriptors = sortDescriptorsAry
request.predicate = predicate
return request
However i get no results. I assume this because both attributes don't match because of the time:
start_date = 2013-08-29 14:27:47 +0000
date = 2013-08-29 17:34:23 +0000
How can i some how tell CoreData to ignore the "177:34:23 +0000" bit, or is there a better way?
Edit:
I do have the option to change the way in which the date format is stored initially:
I have tried this:
var now:NSDate = self.startDatePicker.date
var calendar:NSCalendar = NSCalendar(calendarIdentifier: NSGregorianCalendar)
var components:NSDateComponents = calendar.components(NSCalendarUnit.CalendarUnitYear | NSCalendarUnit.CalendarUnitMonth | NSCalendarUnit.CalendarUnitDay, fromDate: now)
components.hour = 00
components.minute = 00
components.second = 00
var newDate:NSDate = calendar.dateFromComponents(components)!
However on some days, my time is being set to a day before.
For Example:
Selecting the 30th August, after transform I get 2014-08-29 23:00:00 +0000

Answer 1

Before to save start_date attributes in Core Data, you need to be sure that their time is set to 12:00 AM:
//Get "Aug 29, 2014, 12:00 AM" from "Aug 29, 2014, 10:07 PM"
let newDate = NSDate() //or any other date
let calendar = NSCalendar.currentCalendar()
calendar.timeZone = NSTimeZone.systemTimeZone()
var startDate: NSDate?
var duration: NSTimeInterval = 0
calendar.rangeOfUnit(.DayCalendarUnit, startDate: &startDate, interval: &duration, forDate: newDate)
//Create, set and save a new managedObject
//Records, here, is the name of your NSManagedObject subclass
let record = NSEntityDescription.insertNewObjectForEntityForName("Records", inManagedObjectContext: managedObjectContext) as Records
record.start_date = startDate
/* set other attributes here */
var error: NSError?
if !managedObjectContext.save(&error) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
println("Unresolved error \(error), \(error!.userInfo)")
abort()
}
Then, you will be able to fetch a date in Core Data with the predicate you want this way:
//Set your datePicker date to 12:00 AM
let pickerDate = dateFromMyDatePicker //as NSDate
let calendar = NSCalendar.currentCalendar()
calendar.timeZone = NSTimeZone.systemTimeZone()
var predicateDate: NSDate?
var duration: NSTimeInterval = 0
calendar.rangeOfUnit(.DayCalendarUnit, startDate: &predicateDate, interval: &duration, forDate: pickerDate)
//Create your fetchRequest
/* ... */
//Set your predicate
let predicate = NSPredicate(format: "start_date == %#", predicateDate)
Edit
As an alternative to rangeOfUnit:startDate:interval:forDate:, you can use the code you provide. But in both cases, don't forget to add the following line:
calendar.timeZone = NSTimeZone.systemTimeZone()

Answer 2

If you still have to option of changing the way you store data, I would store it on your project-entity as three attributes: an Integer for year, an Integer for month and an Integer for day. It will be easier to code (you can create a predicate like:
NSInteger year = ....
NSInteger month = ....
NSInteger day = ....
[NSPredicate predicateWithFormat: #"day == %# AND month == %# and year == %#", #(day), #(month), #(year)];
) and this probably work quicker compared to using a string. If you want to go with the string (beware extraneous spaces or other interpunction!), you can in code construct a string with the format 'yyyy-mm-dd' (use NSString stringWithFormat:) and then use an NSPredicate like:
NSInteger year = ....
NSInteger month = ....
NSInteger day = ....
NSString * dateString = [NSStringWithFormat: #"%4i-%2i-%2i", year, month, day];
[NSPredicate predicateWithFormat: #"dateString contains %#", dateString];
However, this will only work if YOU constructed the string that you stored, not the NSDate.description(). If you store actual NSDate, see my first original answer :).
But really, if you still have an option, don't store it as a string. It wil really be problematic, if not now, then later (I can tell from experience).

Answer 3

<edit>
Your second way of doing things seems to me like a good way to go. Store the integer values for year, month and day, and create a predicate with that. Your usage of NSDateComponents and calendar is great, and so this should be the easiest way to go. Very important: don't use for the time of 00, as midnight is a tricky time (there is daylight savings times in parts of the world, there is the question of whether exactly midnight is part of the previous of the next day). Just enter 12 as the time, to be as far away as you can from midnight.
Also, i would advice you to watch the WWDC from 2011 'Performing Calendar Calculations, Session 117. It talks among other very interesting things about why midnight is tricky (if not, then that is explained in the 2013 session :) ).
</edit>
NSDate has sub-second accuracy. It's description method (that gets called when you NSLog an NSDate) only displays second-accuracy.
So to use an NSDate in an NSPredicate, always specify 'larger/smaller then or equal to' operators.
For example:
NSDate * lastWeek = // ... created using NSCalendar and NSDateComponents.
NSDate * now = [NSDate date];
NSPredicate * entitiesFromAfterLastWeekAndBeforeNow = [NSPredicate predicateWithFormat: #"start_date >= %# AND start_date <= %#", lastWeek, now];
I would advice against storing a data in something else the a native NSDate, as you might store dates that are incorrect due to conversions. Also, storing dates as string will make queries a lot slower (as strings have to be parsed, and NSDate is simply a number). But that is another discussion, and you might want to ship, so you have to do what you think is best.
Edit: I'm sorry, only now noticed that you are writing Swift. But I think my example is clear enough that it is convertible to Swift, right?