Filtering Realm with nested subqueries

问题

My app has data that looks like this.

class ShelfCollection: Object {
  let shelves: List<Shelf>
}

class Shelf: Object {
  let items: List<Item>
}

class Item: Object {
  var name: String
  let infos: List<String>
}

I'm trying to get all shelves in a shelf collection where any items match the query either by name or by an element in their infos list. From my understanding this predicate should be correct, but it crashes.

let wildQuery = "*" + query + "*"
shelfResults = shelfCollection.shelves.filter(
    "SUBQUERY(items, $item, $item.name LIKE[c] %@ OR SUBQUERY($item.infos, $info, info LIKE[c] %@).@count > 0).@count > 0",
    wildQuery, wildQuery
)

It complies as a NSPredicate, but crashes when Realm is attempting to parse it, throwing me

'RLMException', reason: 'Object type '(null)' not managed by the Realm'

I suspect the nested subquery might be what fails, but I don't know enough about NSPredicate to be sure. Is this an acceptable query, and how can I make it.. work?

回答1:

This is an answer and a solution but there's going to be a number of issues with the way the objects are structured which may cause other problems. It was difficult to create a matching dataset since many objects appear within other objects.

The issue:

Realm cannot currently filter on a List of primitives so this Item property will not work for filtering:

let infos: List<String>

However, you can create another object that has a String property and filter on that object

class InfoClass: Object {
    @objc dynamic var info_name = ""
}

and then the Item class looks like this

class Item: Object {
  var name: String
  let infos = List<InfoClass>()
}

and then you filter based on the InfoClass object, not it's string property. So you would have some objects

let info0 = InfoClass()
info0.info_name = "Info 0 name"

let info1 = InfoClass()
info1.info_name = "Info 1 name"

let info2 = InfoClass()
info2.info_name = "Info 2 name"

which are stored in the Item->infos list. Then the question

I'm trying to get all shelves in a shelf collection...

states you want to filter for a collection, c0 in this case, shelves whose items contain a particular info in their list. Lets say we want to get those shelves whose items have info2 in their list

 //first get the info2 object that we want to filter for
 guard let info2 = realm.objects(InfoClass.self).filter("info_name == 'Info 2 name'").first else {
    print("info2 not found")
    return
}

print("info2 found, continuing")

//get the c0 collection that we want to get the shelves for
if let c0 = realm.objects(ShelfCollection.self).filter("collection_name == 'c0'").first {
    let shelfResults = c0.shelves.filter("ANY items.infoList == %@", info2)
    for shelf in shelfResults {
        print(shelf.shelf_name)
    }
} else {
    print("c0 not found")
}

I omitted filtering for the name property since you know how to do that already.

The issue here is the infos could appear in many items, and those items could appear in many shelf lists. So because of the depth of data, with my test data, it was hard to have the filter return discreet data - it would probably make more sense (to me) if I had example data to work with.

Either way, the answer works for this use case, but I am thinking another structure may be better but I don't know the full use case so hard to suggest that.

来源：https://stackoverflow.com/questions/59414134/filtering-realm-with-nested-subqueries

标签

ios

swift

realm

nspredicate