Is it possible to create NSFetchedResultsController to fetch all entities with distinct values for specific property?

Question

Suppose I have Person entity with properties: id, name, age:

And I have following records:

101, Katherine, 23
102, Madelaine, 23
103, Katherine, 27
104, Vicky, 18
105, Kirsten, 45
106, Vicky, 12

And I need all records sorted by age, without duplicating name.

The output should be:

105, Kirsten, 45
103, Katherine, 27
102, Madelaine, 23
104, Vicky, 18

This is how I create NSFRC:

    let context = NSManagedObjectContext.mr_default()
    let fetchRequest = NSFetchRequest<Person>(entityName: "Person")
    let ageDescriptor = NSSortDescriptor(key: "age", ascending: false)

    fetchRequest.sortDescriptors = [ageDescriptor]

    fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self

How to apply rule to fetch all persons and only one (oldest) person with the same name?

I didn't understand your question. Do you need to search the person based on a string? — Mathi Arasan, Jul 07 '17 at 13:15

AdiTheExplorer · Answer 1 · 2017-07-07T15:39:38.653

0

You can use the property returnsDistinctResults and propertiesToFetchin NSFetchrequest to return only distinct results of a fetch request. In your case the propertiesToFetch will be the name of the entity.

You can find more info here: https://developer.apple.com/documentation/coredata/nsfetchrequest

Just as example code. (I am doing it off top of my head so may not be 100% accurate with exact syntax):

let entity = NSEntityDescription.entity(forEntityName: "Person", in: context)

let entityProperties = entity.propertiesByName
let namePropertyDescription = entityProperties["name"]

fetchrequest.propertiesToFetch = [namePropertyDescription]
fetchrequest.returnsDistinctResults = true

edited Jul 07 '17 at 15:39

answered Jul 07 '17 at 15:08

AdiTheExplorer

259
4
13

name of the entity may be "propertyToFetch"? Really? Unbelieveable:) – Bartłomiej Semańczyk Jul 07 '17 at 15:09
What I meant was you would assign the name field of your entiry to the 'propertiesToFetch' of NSFetchRequest – AdiTheExplorer Jul 07 '17 at 15:13
Please, provide example... I am not sure If I understand tou – Bartłomiej Semańczyk Jul 07 '17 at 15:15

Is it possible to create NSFetchedResultsController to fetch all entities with distinct values for specific property?

1 Answers1