CODEWITHSUNDEEP
ioscore-datafetch
Jakub Turcovsky
Jakub Turcovsky

Reputation: 2126

Core Data: How to get data of entity in relationship?

I have some data in Database. There's a Person and his Addresses as One-to-Many relationship, saved like this:

// Create Person
NSEntityDescription *entityPerson = [NSEntityDescription entityForName:@"Person" inManagedObjectContext:self.managedObjectContext];
NSManagedObject *newPerson = [[NSManagedObject alloc] initWithEntity:entityPerson insertIntoManagedObjectContext:self.managedObjectContext];

// Set First and Last Name
[newPerson setValue:@"Bart" forKey:@"first"];
[newPerson setValue:@"Jacobs" forKey:@"last"];
[newPerson setValue:@44 forKey:@"age"];

// Create Address
NSEntityDescription *entityAddress = [NSEntityDescription entityForName:@"Address" inManagedObjectContext:self.managedObjectContext];
NSManagedObject *newAddress = [[NSManagedObject alloc] initWithEntity:entityAddress insertIntoManagedObjectContext:self.managedObjectContext];

// Set info
[newAddress setValue:@"Main Street" forKey:@"street"];
[newAddress setValue:@"Boston" forKey:@"city"];

// Add Address to Person
[newPerson setValue:[NSSet setWithObject:newAddress] forKey:@"addresses"];

// Create Address
NSManagedObject *otherAddress = [[NSManagedObject alloc] initWithEntity:entityAddress insertIntoManagedObjectContext:self.managedObjectContext];

// Set info
[otherAddress setValue:@"5th Avenue" forKey:@"street"];
[otherAddress setValue:@"New York" forKey:@"city"];

// Add Address to Person
NSMutableSet *addresses = [newPerson mutableSetValueForKey:@"addresses"];
[addresses addObject:otherAddress];

// Save Managed Object Context
NSError *error = nil;
if (![newPerson.managedObjectContext save:&error]) {
    NSLog(@"Unable to save managed object context.");
    NSLog(@"%@, %@", error, error.localizedDescription);
}

Now I need to fetch this person. I can fetch all of his attributes, but how should I get his addresses and attributes of them?

My code so far:

// Fetching
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Person"];

// Execute Fetch Request
NSError *fetchError = nil;
NSArray *result = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];

if (!fetchError) {
    for (NSManagedObject *managedObject in result) {
        NSLog(@"%@, %@", [managedObject valueForKey:@"first"], [managedObject valueForKey:@"last"]);
        // HERE I WANT TO PRINT INFO ABOUT HIS ADDRESSES
    }

} else {
    NSLog(@"Error fetching data.");
    NSLog(@"%@, %@", fetchError, fetchError.localizedDescription);
}

Upvotes: 0

Views: 172

Answers (1)

Mundi
Mundi

Reputation: 80271

Just fetch the Person. Fetching the attributes will be done for you in an optimized fashion once you need the attributes. You simply access the attributes (assume they are already there).

NSString *message = [NSString stringWithFormat:@"Hello, %@.", person.first];

If you need the addresses, you get them with 

NSSet *addresses = person.addresses;

They are in no particular order, but you can sort them with sortedArrayUsingDescriptors and similar methods. In any case, you do not need another fetch request.

BTW, your way to set the relationship is unnecessarily complicated and error prone. If one side of the relationship is to-one, use it to define the relationship: 

newAddress.person = person;

Upvotes: 1

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