Pheepster Pheepster - 2 months ago 16
Objective-C Question

Handling duplicate entries in Core Data

I have an app that allows users to save favorites. I am using Core Data to store the favorites as managed objects. I have written some code to prevent the possibility of storing duplicates, but am wondering if there is a better way to do so. Each favorite object has an ID field that is unique. In the following code I am simply looping through and checking the ID field, and if the value already exists, setting a flag value to true, and breaking out of the loop.

-(BOOL)addFavorite{
BOOL entityExists = NO;
if(context){
// does this favorite already exist?
NSArray *allFaves = [AppDataAccess getAllFavorites];
for(Favorite *f in allFaves){
if([f.stationIdentifier isEqualToString:stID]){
entityExists = YES;
break;
}
}
if(!entityExists){
NSError *err = nil;
Favorite *fave = [Favorite insertInManagedObjectContext:context];
fave.stationRealName = riverGauge.name;
fave.stationIdentifier = stID;
fave.stationState = @"WV";
if(![context save:&err]){
NSLog(@"ERROR: Could not save context--%@", err);
}
return YES;
}
return NO;
}


I was wondering if Core Data has the ability to check to see if an object being added is a duplicate. Is there a predicate that can handle checking for duplicates? Thanks!

Answer

CoreData does no uniquing by itself. It has no notion of two entries being identical.

To enable such a behavior you have to implement it yourself by doing a 'search before insert' aka a 'fetch before create'.

NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Favorite"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"stationIdentifier == %@", stID];
[fetch setPredicate:predicate];
YourObject *obj = [ctx executeRequest:fetch];

if(!obj) {
    //not there so create it and save
    obj = [ctx insertNewManagedObjectForEntity:@"Favorite"]; //typed inline, dont know actual method
    obj.stationIdentifier = stID;
    [ctx save];
}

//use obj... e.g.
NSLog(@"%@", obj.stationIdentifier);

Remember this assumes single-threaded access

Comments