CODEWITHSUNDEEP
xcodecore-data
Peter C. Glade
Peter C. Glade

Reputation: 629

Filtering CoreData Fetch with parentEntities

I have a Problem with my CoreData Model: enter image description here

If i have a lot of TopObjects and want to fetch all SubObject's which have a relationship to a specific object, how can i filter my results in the fetch predicate. normally i would set a predicate like "top = refObject". but the abstract entity SubObject has no relationship "top", just the entities itself.

If i try to add the relationship only to the parent entity "SubObject" i lost the direct Relations in the TopEntity.

Can anybody give me a hint ?

Upvotes: 0

Views: 97

Answers (1)

ospr
ospr

Reputation: 1671

Not sure if there's a way of conditionally specifying a relationship in the predicate (which would allow a single fetch request to do this), but below is possibly a way to fetch the objects you need in multiple fetches. The idea is to iterate through all of the entities in the managed object model and check if they have the TopObject relationship and are of the SubObject class, then fetch them based on the topObject.

for (NSEntityDescription *entityDescription in managedObjectModel)
{
    // Attempt to pull out the TopObject relationship
    NSRelationshipDescription *topRelationshipDescription = entityDescription.relationshipsByName[@"top"];

    // Test if the relationship points to the TopObject and if the entity is of the correct class
    if ([topRelationshipDescription.destinationEntity.name isEqualToString:@"TopObject"] &&
        [NSClassFromString(entityDescription.managedObjectClassName) isSubclassOfClass:[SubObject class]])
    {
        NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:entityDescription.name];
        fetchRequest.predicate = [NSPredicate predicateWithFormat:@"top = %@", topObject];

        // fetch objects and add them to an array
    }
}

Upvotes: 0

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