Query MongoDB Realm array only at first index - swift

Question

I need to query MongoDB Realm from synced iOS and Android app. In Swift I can write something like this:
let dictionary = realm?.objects(myReamlObject.self)
let results = dictionary?.where {
$0.senses.glosses.term == "the term I want"
}
or using predicate:
let results = dictionary?.filter("ANY senses.glosses.term == %#", "the term I want")
Both work well, but I don't want to check ALL senses.glosses.term.
Every entry has (or could have) many senses and many glosses.
I would like to check term of first senses in first glosses only.
Something I would write like this:
let results = dictionary?.where {
$0.senses[0].glosses[0].term == "the term I want"
}
But it gives error:
Referencing subscript 'subscript(_:)' on 'Query' requires that
'List<myRealmObject_senses>' conform to 'RealmKeyedCollection'
Any suggestion on how to query only first index of an array in MongoDB Realm? Thank you

Answer 1

Let me re-state the question
How to query a Realm objects' List property - but only query on the first
element in the List.
The answer is going to depend on the amount of results being worked with.
Here's how to do it in a way that's O.K. for small datasets but NOT RECOMMENDED
Your models were not included in the question so let me use a simplified model of a PersonClass that as a List of DogClass objects
class PersonClass: Object {
#Persisted var name = ""
#Persisted var dogList = List<DogClass>()
}
class DogClass: Object {
#Persisted var name = ""
}
The idea here is to use Swift high-level functions to only test the first item in each persons doglist for a match (this can be applied to other languages as well)
//get all the people
let peopleResults = realm.objects(PersonClass.self)
//use the high-level Swift function compactMap to return all of
// the people whose first dog is named "Spot"
let persons = peopleResults.compactMap { person -> PersonClass? in
if person.dogList.first?.name == "Spot" {
return person
}
return nil
}
The downside is that this code overrides a fundamental advantage of Realm - that Realm objects are lazily loaded if Realm functions are used e.g. as soon as a high-level Swift function is used, ALL of the objects are loaded into memory, potentially overwhelming the device.
A better option is to simply add a managed property to the PersonClass that also points to the 0'th element in the list.
class PersonClass: Object {
#Persisted var name = ""
#Persisted var dogList = List<DogClass>()
#Persisted var mainDog: DogClass?
func addMainDog(withDog: DogClass) {
self.dogList.append(withDog)
self.mainDog = withDog
}
}
as you can see, there's also a function to add that first dog to the list and also populates the mainDog property which points to the same object. It's one property so the overall impact is very low but the advantages for simple queries are very high.
From there the query becomes trivial
let peopleResults = realm.objects(PersonClass.self).where { $0.mainDog.name == "Spot" }
Expanding on this, you could save the 0th element of each List object in the Parent object or even have a property in each child object that points to the first element in it's respective list.