Delete an object in core data

Question

I have an entity in my core data model like this:

@interface Selection : NSManagedObject

@property (nonatomic, retain) NSString * book_id;
@property (nonatomic, retain) NSString * contenu;
@property (nonatomic, retain) NSNumber * page_id;
@property (nonatomic, retain) NSNumber * nbrOfOccurences;
@property (nonatomic, retain) NSString * next;
@property (nonatomic, retain) NSString * previous;

I have created many Selections and saved them in Core Data and now I would like to delete some selections with some criteria. For example, I would like to delete a Selection object if matches the following:

content = test
page_id = 5
book_id = 1331313

How I can do this?

Answer 1

What Mike Weller wrote is right. I'll expand the answer a little bit.

First you need to create a NSFetchRequest like the following:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];    
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Selection" inManagedObjectContext:context]];

Then you have to set the predicate for that request like the following:

[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"content == %@ AND page_id == %@ AND book_id == %@", contentVal, pageVal, bookVal]];

where

NSString* contentVal = @"test";
NSNumber* pageVal = [NSNumber numberWithInt:5];
NSString* bookVal = @"1331313";

I'm using %@ since I'm supposing you are using objects and not scalar values.

Now you perform a fetch in the context with the previous request:

NSError* error = nil;
NSArray* results = [context executeFetchRequest:fetchRequest error:&error];

results contains all the managed objects that match that predicate.

Finally you could grab the objects and call a deletion on them.

[context deleteObject:currentObj];

Once done you need to save the context as per the documentation.

Just as a new object is not saved to the store until the context is saved, a deleted object is not removed from the store until the context is saved.

Hence

NSError* error = nil;
[context save:&error];

Note that save method returns a bool value. So you can use an approach like the following or display an alert to the user. Source NSManagedObjectContext save error.

NSError *error = nil;
if ([context save:&error] == NO) {
    NSAssert(NO, @"Save should not fail\n%@", [error localizedDescription]);
    abort();
}