CODEWITHSUNDEEP
iosswiftxcodecore-data
konyv12
konyv12

Reputation: 776

'to-many key not allowed here' exception but why?

I am building a maps app with the following entities:

enter image description here

enter image description here

Coordinate to Map is to-one and Map to Coordinate is a to-many relationship.

And my code would be:

 fileprivate var fetchedResultsControllerForMapEntity: NSFetchedResultsController<Map> = {
// Create Fetch Request
let fetchRequest: NSFetchRequest<Map> = Map.fetchRequest()

// Configure Fetch Request
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
fetchRequest.predicate = NSPredicate(format:"newRelationship == %@", "peachesfarm")

// Create Fetched Results Controller
let fetchedResultsControllerForMapEntity = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

return fetchedResultsControllerForMapEntity
}()

fileprivate var fetchedResultsControllerForCoordinateEntity: NSFetchedResultsController<Coordinate> = {
// Create Fetch Request
let fetchRequest: NSFetchRequest<Coordinate> = Coordinate.fetchRequest()

// Configure Fetch Request
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "latitude", ascending: true)]
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "longitude", ascending: true)]

// Create Fetched Results Controller
let fetchedResultsControllerForCoordinateEntity = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

return fetchedResultsControllerForCoordinateEntity
}()

I am basically trying to retrieve the coordinates of the map called "peachesfarm" ONLY. Why might I be getting this exception?

Upvotes: 0

Views: 2015

Answers (1)

Martin R
Martin R

Reputation: 539805

NSPredicate(format:"newRelationship == %@", "peachesfarm")

gives an error because "newRelationship" is a to-many relationship, which cannot be compared for equality.

To fetch all coordinates which are related to a map with a given name, you have to use a fetch request for "Coordinate", not for "Map". Then use the key path to the name of the related map. Using the relationship names as suggested by @vadian above that would be

let fetchRequest: NSFetchRequest<Coordinate> = Coordinate.fetchRequest()
let predicate = NSPredicate(format: "map.name == %@", "peachesFarm")
fetchRequest.predicate = predicate

Even better, use the #keyPath directive, where the compiler checks the property names so that errors are avoided:

let predicate = NSPredicate(format: "\(#keyPath(Coordinate.map.name)) == %@", "peachesFarm")

Upvotes: 2

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