Filter realm objects to only get one (distinct) object by attribute

问题

Let me explain first cause title may be a bit confusing.

Say I have this realm objects of type Movie:

Movie1(id: 0, genre: "horror")

Movie2(id: 1, genre: "horror")

Movie3(id: 3, genre: "sci-fi")

What I need to do is get the first for every genre (in this case Movie1 and Movie3)

I'd like to do it w/o loops using only realm + NSPredicate, so the performance is better, but I'm a bit stuck there...

So far what I got is this:

Realm().objects(Movie.self).sorted(byKeyPath: id, ascending: true)
.value(forKeyPath: "@distinctUnionOfObjects.genre") 

This returns me an array with ("horror", "sci-fi") and I can't really figure what should be the next step. If I try to do a:

 .filter("genre IN %@", arrayWithDistinctGenres) 

it will return me all the objects.

Probably it's simple but I cannot figure how to do it.

Any help is appreciated.

回答1:

As of Realm Swift 3.1 you can do let movies = realm.objects(Movie.self).sorted(by: ["id", true]).distinct(by: ["genre"]), which will select the first Movie of each genre using the sort order applied before the distinct operation.

回答2:

You need to filter by one genre at a time. Something like this

let genres = realm.objects(Genres.self)
let movies = realm.objects(Movie.self).sorted(byKeyPath: id, ascending: true)

for genre in genres {
    let firstInThisGenre = movies.filter({ $0.genreId == genre.id }).first
}

来源：https://stackoverflow.com/questions/48742646/filter-realm-objects-to-only-get-one-distinct-object-by-attribute