Question

I have a relation in Core Data like this:

A -> B

and the following tables:

A
----
id 
path
ref_foreignkey_B

B
----
id
name

The problem is that the foreign key is set as B object (inherits from NSObject).

Then I am trying to fetch all entries from A where a reference to a certain B object is set. I know the object B but cannot figure out how to set the NSPredicate correctly.

I have tried the following:

NSString *complexPredicateFormat = [NSString stringWithFormat:@"ZREF_B == %@", self.B];   
NSPredicate *filter = [NSPredicate predicateWithFormat:complexPredicateFormat];
[fetchRequest setPredicate:filter];

… but that results in the following error:

'NSInvalidArgumentException', reason: 'Unable to parse the format string "ZREF_B == <B: 0x89d8300> (entity: B; id: 0x89d3ca0 <x-coredata://...

Does anyone know how to set a NSManagedObject (updated) as predict filter (represents the where clause)?

EDIT:
I am sorry. Of course A and also B inherits from NSManagedObject and NOT NSObject.

Answer 1

Based on Gabriel suggestion:

Have you created a Data Model? Why your entities subclass NSObject?

Then

If ref_foreignkey_B is a relationship for entity B you need to use

[NSPredicate predicateWithFormat:@"ref_foreignkey_B == %@", self.B];

Obviously if you use that predicate you need to query (use a fetch request) against A and not B.

NSFetchRequest* fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"A"];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"ref_foreignkey_B == %@", self.B];

Some Notes

Why do you use ZREF_B? This is the column created in the sqlite file (I suppose). You don’t have to use that but only properties (attributes or relationships) declared in the data model.

id, path and ref_foreignkey_B for A

id and name for B

When you deal with Core Data you deal with an object graph and not with the db (sqlite for example) itself. This is an important point to understand. You have not to deal with the sqlite file (for example) that is created for you. Only with the data model you have created.

In addition, I suggest you to create an inverse relationship from B to A. This lets you to maintain integrity in the object graph. For info read Relationships.

Hope it helps.