How to get distinct results from a single field in Core Data (Swift 4)

Question

I'm trying to get a distinct list of all of the values from the "flightNumber" field in my "LogLine" table. But everything I've tried has resulted in the fetch request returning the full list of flight numbers, with duplicates.

I've followed the answers from these questions:

Swift Core Data - Request with distinct results

Swift 3 - NSFetchRequest Distinct Results

But can't seem to get it to work. Here is my code:

func fetchUniqueFlightNumbers() -> [[String: Int16]]? {
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "LogLine")
    request.resultType = NSFetchRequestResultType.dictionaryResultType
    request.returnsDistinctResults = true
    request.propertiesToFetch = ["flightNumber"]

    do {
        let results = try persistenceContainer.viewContext.fetch(request) as! [[String: Int16]]
        return results
    } catch {
        print("Couldn't read flight numbers from DB \(error)")
        return nil
    }
}

The results I'm getting are:

[["flightNumber": 1], ["flightNumber": 1], ["flightNumber": 2], ["flightNumber": 2]]

I want to get the result [1,2] but I'm getting [1,1,2,2].

The attribute "flightNumber" is an Integer 16.

Is there something wrong with my code, or has something changed in Swift 4?

EDIT:

I realized that I'm only seeing this behavior in testing, when my persistent store is configured as an NSInMemoryStoreType. So it isn't as much of a problem, I'll just have to rethink the unit tests for this part of the code. I am curious why I'm seeing this difference in behavior between the two store types though.

That sure looks correct. What type is the `flightNumber` attribute? — Tom Harrington, Mar 07 '18 at 18:05
Don't know then, I tried the same thing and got the expected results. — Tom Harrington, Mar 07 '18 at 23:31
Thanks for testing that. I tried it again today and realized that I'm only seeing this behavior in testing, when using an in-memory persistent store. When I use the default store it's working as expected — Chris.B, Mar 08 '18 at 15:03
Have you gotten any feedback from Apple? Also, you may wish to add this bug to OpenRadar: https://openradar.appspot.com — ThomasW, Apr 06 '18 at 07:18
hey @Chris.B I landed on your post after struggling with this for a while and finally realised the issue only affects unit tests with in memory contexts. In my case using Swift 4.1 (and 4.2 beta) my fetch returns no entries whatsoever. Have you heard back from Apple on this yet? — Rog, Jul 28 '18 at 20:58

score 0 · Answer 1 · answered May 19 '19 at 10:12

@Chris.B I see your post is a bit old but unsolved. I just went through a similar problem but using FethchedResultControllerand I found out a lot. The problem you're having is that you're not using .propertiesToGroupBy and this is the properties you want distinct results of. It is a mandatory NSFetchRequest property and must be used in order to get distinct results. For a complete insight check my answer at my own question as is a very detailed sort of step-by-step guide. Removing duplicate objects from fetch based on object parameter UPDATED Swift Hope it helps.

How to get distinct results from a single field in Core Data (Swift 4)

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