Core-data many-to-many relationship - ios

Question

I have an Android app, and now I'm making an iOS version, but I'm having some problem with the joins in CoreData.
I have the following tables:
Cidade
-cid_codigo integer primary key
-cid_nome text
-cid_nome_normalizado text
Anunciante
-anu_codigo integer primary key
-anu_nome text
-some other values
AnuncianteCidade
-cid_codigo integer
-anu_codigo integer
To get the all data from table Cidade I use the following method:
+(NSMutableArray *)getAllCidades{
NSMutableArray *retorno = [[NSMutableArray alloc] init];
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [appDelegate managedObjectContext];
NSEntityDescription *entityDesc = [NSEntityDescription entityForName:#"Cidade" inManagedObjectContext:context];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entityDesc];
//WHERE CLAUSE
NSPredicate *pred = [NSPredicate predicateWithFormat:#"1 = 1"];
[request setPredicate:pred];
NSError *error;
NSArray *cidades = [context executeFetchRequest:request error:&error];
if([cidades count] == 0){
NSLog(#"Nenhuma cidade encontrada");
}else{
for(NSManagedObject *cidade in cidades){
Cidade *c = [[Cidade alloc] init];
[c initWithCodigo:[[cidade valueForKey:#"cid_codigo"] integerValue] nome:[cidade valueForKey:#"cid_nome"] nomeNormalizado:[cidade valueForKey:#"cid_nome_normalizado"]];
[retorno addObject:c];
}
}
return retorno;
}
But now, given a name from Cidade, I want to get all the data from Anunciante associated with this Cidade.
How can I do that?

Answer 1

Core Data is not a database. Core Data is an object graph that happens to persist to disk and one of the formats that Core Data can persist to is a database structure. This is an important distinction that will help you to work with it moving forward.
First, you cannot call just -init on a NSManagedObject. That will not work as it is not the designated initializer of NSManagedObject. I would suggest you read up on Core Data and learn how to stand up the Core Data stack.
Having said that, your Cidade objects should have a reference to Anunciante. The join table is internal to Core Data and you don't have access to it nor should you. To get all of the Anunciante objects for a Cidade object is to simply request the objects:
Given an NSArray of Cidade objects:
NSArray *objects = ...;
for (NSManagedObject *object in objects) {
NSSet *anunciantes = [object valueForKey:#"anunciante"];
}
This is assuming you have a many to many relationship defined in the Core Data model editor between the Cidade and the Anunciante entities.

Answer 2

In addition to Marcus' answer, I would add that a predicate "1 = 1" could be simply left out.
To insert a managed object into the context you use a NSEntityDescription class method:
Cidade *cidade = [NSEntityDescription insertNewObjectForEntityForName:#"Cidade"
inManagedObjectContext:context];
All "anunciantes" of a cidade will be conveniently available to you as a NSSet:
cidade.anunciantes
is all you need.