Removing a specific entry/row from Core-Data

Question

I'm using core data in my app, and i'm confused when it comes to removing certain rows or entries from the core data storage. I insert some products in to the storage like so:

NSManagedObject *Product = [NSEntityDescription insertNewObjectForEntityForName:@"Product" inManagedObjectContext:context];
[Product setValue:[NSNumber numberWithFloat:id] forKey:@"pid"];
[Product setValue:[NSNumber numberWithFloat:quantity] forKey:@"pquantity"];

This works fine for insertion. However, later in the app, I want to remove the entry where for example, the pid is 53. How would I go about removing only this row/entry? The equivalent SQL would be something like:

DELETE from Product WHERE pid = '53'

I would greatly appreciate some example code, as I can't seem to figure this one out.

Thanks for any help.

Solution 1

As @Nektarios said, you are dealing with objects here so you want to find an object that has a particular attribute value. You that with a fetch request and a predicate.

  NSNumber *soughtPid=[NSNumber numberWithInt:53];
  NSEntityDescription *productEntity=[NSEntityDescription entityForName:@"Product" inManagedObjectContext:context];
  NSFetchRequest *fetch=[[NSFetchRequest alloc] init];
  [fetch setEntity:productEntity];
  NSPredicate *p=[NSPredicate predicateWithFormat:@"pid == %@", soughtPid];
  [fetch setPredicate:p];
  //... add sorts if you want them
  NSError *fetchError;
  NSArray *fetchedProducts=[self.moc executeFetchRequest:fetch error:&fetchError];
  // handle error

The fetchedProducts array will contain all the objects of the entity Product whose pid attribute equals soughtPid. Note that the predicate fulfills the same function logically as the where clause in SQL.

Once you have the objects you just tell the context to delete them:

  for (NSManagedObject *product in fetchedProducts) {
    [context deleteObject:product];
  }

When you next save the context, the object's data will be deleted from the persistent store file.

Solution 2

Like Sqlite's "DELETE from tableName WHERE condition", you have no single step to delete MULTIPLE objects from CoreData.

In CoreData, first you have to fetch objects that has to be deleted, using NSFetchRequest and NSPredicate

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"EntityName"];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"propertyName == %@", value];
[request setPredicate:predicate];

NSError *error = nil;
NSManagedObjectContext *managedObjectContext;//Get your ManagedObjectContext;
NSArray *result = [managedObjectContext executeFetchRequest:request error:&error];

Then, you should iterate through every NSManagedObject and call deleteObject: from your NSManagedObjectContext.

if (!error && result.count > 0) {
    for(NSManagedObject *managedObject in result){
        [managedObjectContext deleteObject:managedObject];
    }

    //Save context to write to store
    [managedObjectContext save:nil];
}

Solution 3

Use this method:

+ (void)Deletebeer:(NSString*)m_Beerid
{
    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [appDelegate managedObjectContext];

    NSEntityDescription *productEntity=[NSEntityDescription entityForName:@"Beers" inManagedObjectContext:context];
    NSFetchRequest *fetch=[[NSFetchRequest alloc] init];
    [fetch setEntity:productEntity];
    NSPredicate *p=[NSPredicate predicateWithFormat:@"m_Beerid == %@", m_Beerid];
    [fetch setPredicate:p];
    //... add sorts if you want them
    NSError *fetchError;
     NSError *error;
    NSArray *fetchedProducts=[context executeFetchRequest:fetch error:&fetchError];
    for (NSManagedObject *product in fetchedProducts) {
        [context deleteObject:product];
    }
    [context save:&error];
}

Solution 4

Remember to think of Core Data as a graph and not a DB, so the concept of rows doesn't apply. Instead, you want to remove a specific object from the graph.

You used insertNewObjectForEntityForName:inManagedObjectContext: to insert it. Use deleteObject: to delete it, such as:

[aContext deleteObject:aManagedObject];

In your case,

[context deleteObject:Product];

Good luck

See Apple's explanation here

Note: when you delete, depending on your scheme, that can have different implications. For example it could delete everything further down the child-path of your Core Data object graph. Make sure you think hard when designing your schema about how that should work. If you set it up right, this can be a huge advantage to you.

Solution 5

use this

Entity  *entity = (Entity *)[NSEntityDescription insertNewObjectForEntityForName:@"Entity" inManagedObjectContext:managedObjectContext];
[entity setTimeStamp:[NSDate date]];
NSError *error;
if ([managedObjectContext save:&error]) {

}
[eventArray delete:<#(id)sender#>];

[self.tableView reloadData];

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Comments

[MahboobiOSDeveloper]

I edit some code according to my requirement but this code works for me thanks @TechZen

[amish]

This code doesn't remove the row, but instead makes the currentItemId = 0. I am searching to delete the row instead.