Get object by objectID?

[gerchicov-bp]

I need to save objectID and through some time get it (or nil if it doesn't exist) by ID. The problem is I should store objectID as string but its type is NSManagedObjectID. Could you explain how to save this unique id and get an object with this id according to this library?

[kevnm67]

@gerchicov-bp
It's no different from using core data. You can store the absolute string of an objectID's URI representation. An example of using a category on RHCoreDataTableViewController is one example accomplishing your goal.

@implementation RHCoreDataTableViewController (KJMAdditions)

- (NSString *)kjm_objectIDstringRepresentation
{
    return [self kjm_selectedObjectID].URIRepresentation.absoluteString;
}

- (NSManagedObjectID *)kjm_selectedObjectID
{
    NSManagedObject *selectedObject = [self.fetchedResultsController objectAtIndexPath:(self.tableView).indexPathForSelectedRow];

    return selectedObject.objectID;
}

@end

Also, you should make sure the ObjectID is a permanentID before saving it.

- (NSManagedObjectID *)kjm_permanentObjectID
{
    NSError *error = nil;

    if (![self.managedObjectContext obtainPermanentIDsForObjects:@[self] error:&error]) {
        NSLog(@"Couldn't obtain a permanent ID for object %@", error);
    }

    return self.objectID;
}

[gerchicov-bp]

I did it slightly differently:

NSManagedObjectID *objID = [managedObjectContext.persistentStoreCoordinator managedObjectIDForURIRepresentation:[NSURL URLWithString:objIDString]];
id obj = [managedObjectContext existingObjectWithID:objID error:&err];

Could you explain the last step with permanentID?

[kevnm67]

The last step is when saving a managed object id used to find objects later on, as you said in the first post. From apple docs:
"New objects inserted into a managed object context are assigned a temporary ID which is replaced with a permanent one once the object gets saved to a persistent store."

[gerchicov-bp]

Ok, thanks it may help to solve my another problem. Additionally maybe do u know how to operate with objectID in overridden willSave and didSave methods if this id may be invalid?

[kevnm67]

Not sure I exactly understand what you're trying to do but keep this in mind:
Also, check apple's documentation and compare the two methods below.

// It never returns nil, and never performs I/O.  The object specified by objectID is assumed to exist, and if that assumption is wrong the fault may throw an exception when used.
- (NSManagedObject *)objectWithID:(NSManagedObjectID *)objectID;

// vs.
// Returns nil if it does not exist
- (nullable NSManagedObject *)existingObjectWithID:(NSManagedObjectID*)objectID error:(NSError**)error;

Also, if your passing the objectID, using a segue, for example, you can check whether the objectID exists --

if (self.yourObjectIDProperty) { // fetch the object using the objectID };

[gerchicov-bp]

ok. My task is to save/delete objects from core spotlight. The problem is core spotlight is "writeonly". So I decided to create/update/delete data from core spotlight simultaneously with core data (willSave and didSave methods). Now it works somehow but If this problem occurs again I'll have duplicate entries in core spotlight because I use entity ID as unique identifier (core spotlight stores values for unique keys similarly to NSDictionary but you can't read from it previously stored data)