Deleting objects in core data depending on JSON - ios

Question

I have a NSManagedObjectModel with an entity, "Project". I get all the projects to show them in a UITableView with NSFetchedResult controller. Now if the JSON has new Projects I insert them and if the JSON has updated items I update the items in the Core Data context.
So, my problem is when I get a JSON with less items than the context. I have thought about two ways to delete items in my context. One way is Delete all the context and save it again with the new items. The other way is create an array with all the items in the context and check it with the items in the JSON by id and if there is not one item, then remove it.
I have the idea, but I don't know which is the best way. I have thought also in a backgroundContext.
I use this method right now with out deleting methods:
#pragma mark - Project List service
- (void)getProjectListWithCpompletionBlock:(CompletionBlock)completionBlock{
NSMutableURLRequest *request = [self requestWithMethod:#"GET" path:kAPIProjectList parameters:nil];
[request setTimeoutInterval:kTimeOutRequest];
AFJSONRequestOperation *requestOperation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
NSDictionary *projects = [JSON valueForKey:kTagProjects];
for (NSDictionary *projectDic in projects) {
Project *pro = [Project getProjectWithId: [projectDic objectForKey:kTagProjectId] ];
if (pro) {
[Project updateProjectWithDictionary:projectDic];
NSLog(#"update %# ",[projectDic objectForKey:kTagProjectId]);
} else {
[Project createProjectWithDictionary: projectDic];
NSLog(#"create %# ",[projectDic objectForKey:kTagProjectId]);
}
}
[ypCoreDataManager saveContext];
if (completionBlock) {
completionBlock(NO, nil);
}
}
failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *aError, id JSON) {
NSLog(#"%# Failure JSNON Error%#", NSStringFromSelector(_cmd), aError);
if (completionBlock) {
completionBlock(YES, aError);
}
}];
[self enqueueHTTPRequestOperation:requestOperation];
}
Project+Helper is my project category and here it is the code.
+ (Project *)createProjectWithDictionary:(NSDictionary *)dic {
Project *project = nil;
project = [NSEntityDescription insertNewObjectForEntityForName:#"Project" inManagedObjectContext:mainContext];
project.projectId = [NSNumber numberWithInt:[[dic valueForKey:kTagProjectId] intValue]];
project.title = [[dic valueForKey:kTagProjectTitle]description];
project.estimatedPrice = [NSNumber numberWithInt:[[dic valueForKey:kTagProjectEstimatedPrice] floatValue]];
NSMutableArray *tags = [[NSMutableArray alloc] init];
tags = [dic objectForKey:kTagProjectsTags];
NSMutableSet *tagSet = [[NSMutableSet alloc]init];
for (NSDictionary * tagDic in tags){
NSString *tagName = [tagDic objectForKey:kTagProjectTagName];
Tag *tag = [Tag insertTagName:tagName inManagedObjectContext:mainContext];
[tagSet addObject:tag];
}
[project addTags:tagSet];
return project;
}
// Return project by id
+ (Project *)getProjectWithId:(NSString *) projectId {
Project *project = nil;
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Project"];
request.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"projectId" ascending:YES]];
request.predicate = [NSPredicate predicateWithFormat:#"projectId = %#", [projectId description]];
// Execute the fetch
NSError *error = nil;
NSArray *matches = [mainContext executeFetchRequest:request error:&error];
if (!matches || ([matches count] > 1)) { // nil means fetch failed; more than one impossible (unique!)
// handle error
} else { // found the Project, just return it from the list of matches (which there will only be one of)
project = [matches lastObject];
}
return project;
}
// Update project
+ (Project *)updateProjectWithDictionary:(NSDictionary *)dic {
Project *project = nil;
// Build a fetch request to see if we can find this Project in the database.
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Project"];
request.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"title" ascending:YES]];
request.predicate = [NSPredicate predicateWithFormat:#"projectId = %#", [dic[kTagProjectId]description]];
// Execute the fetch
NSError *error = nil;
NSArray *matches = [mainContext executeFetchRequest:request error:&error];
// Check what happened in the fetch
if (!matches || ([matches count] > 1)) { // nil means fetch failed; more than one impossible (unique!)
// handle error
} else {
project = [matches lastObject];
project.projectId = [NSNumber numberWithInt:[[dic valueForKey:kTagProjectId] intValue]];
project.title = [[dic valueForKey:kTagProjectTitle]description];
project.estimatedPrice = [NSNumber numberWithInt:[[dic valueForKey:kTagProjectEstimatedPrice] floatValue]];
NSMutableArray *tags = [[NSMutableArray alloc] init];
tags = [dic objectForKey:kTagProjectsTags];
NSMutableSet *tagSet = [[NSMutableSet alloc]init];
for (NSDictionary * tagDic in tags){
NSString *tagName = [tagDic objectForKey:kTagProjectTagName];
Tag *tag = [Tag insertTagName:tagName inManagedObjectContext:mainContext];
[tagSet addObject:tag];
}
[project addTags:tagSet];
}
return project;
}

Answer 1

You have to add this method in your Project's category and in your code after you add the new item call this method where you pass you array objects living in the Core Data and it remove all the objects more that you haven't in the array
+(void)removeExpiredProjectBy:(NSMutableArray *)ProjectLiving inContext:(NSManagedObjectContext *)context{
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Project"];
if (projectLiving.count) {
request.predicate = [NSPredicate predicateWithFormat:#"NOT (projectId IN %#)", [projectLiving copy]];
NSError *error = nil;
NSArray *matches = [context executeFetchRequest:request error:&error];
if (matches.count != 0) {
for (Project *pro in matches) {
[context deleteObject:pro];
}
}
}
}

Answer 2

As you process the JSON you can build a list of all of the ids that have been added / updated. Then, after that is complete you can create a fetch request with a predicate which finds all items where NOT (id IN %#) and supply the list of ids. This will return you only the items that need to be deleted.
Or, from an efficient API point of view, the server should give you a deletion list because it doesn't need to confirm items that haven't changed if you send it a 'last request date'...