iOS Realm Filter objects in a list of a relationship

Question

I have three objects nested via lists like this:

class Canteen: Object {

        dynamic var name: String?
        let lines = List<Line>()
}

class Line: Object {

        dynamic var name: String?
        let meals = List<Meal>()
}

class Meal: Object {

        dynamic var name: String?
        dynamic var vegan: Bool = false
}

Getting all canteens with all the lines and meals is no problem. What im doing right now is this:

let predicate = NSPredicate(format: "name == %@", selectedCanteenType.rawValue)
canteens =  realm.objects(Canteen).filter(predicate)

But now i only need the meals which are vegan. So im looking to get the selected canteen with all the lines, but only with meals which are vegan. Is this possible in realm, to filter lists in retrieved objects?

Answer 1

Realm doesn't have any sort of concept of a deep-filtered view, so you can't have a Results<Canteen> which restricts the Lists contained in related objects to vegan meals.

There are several similar things which you can do. You could add inverse relationship properties, and then query Meal objects instead:

class Canteen: Object {
    dynamic var name: String?
    let lines = List<Line>()
}

class Line: Object {
    dynamic var name: String?
    let meals = List<Meal>()
    let canteens = LinkingObjects(fromType: Canteen.self, property: "lines")
}

class Meal: Object {
    dynamic var name: String?
    dynamic var vegan: Bool = false
    let lines = LinkingObjects(fromType: Line.self, property: "meals")
}

let meals = realm.objects(Meal).filter("vegan = true AND ANY lines.canteens.name = %@", selectedCanteenType.rawValue)

(Or rather, you will be able to once Realm 0.102.1 is out; currently this crashes).

If you just need to iterate over the meals but need to do so from the Canteen down, you could do:

let canteens = realm.objects(Canteen).filter("name = %@ AND ANY lines.meals.vegan = true", selectedCanteenType.rawValue)
for canteen in canteens {
    for line in canteen.lines.filter("ANY meals.vegan = true") {
        for meal in line.meals.filter("vegan = true") {
            // do something with your vegan meal
        }
    }
}

This unfortunately has some duplication due to needing to repeat the filter for each level of the references.

Answer 2

Try this:

let predicate = NSPredicate(format: "name == %@", "")
var canteens: [Canteen] = realm.objects(Canteen).filter(predicate).map { can in
    // Iterate through all the Canteens
    let lines: [Line] = can.lines.map { (line: Line) in
        // Iterate through all the lines in each canteene
        // Filter all the Meals that are NOT vegan
        let meals = line.meals.filter { $0.vegan == true }
        line.meals = List<Meal>(meals)
        return line
    }
    can.lines = List<Line>(lines)
    return can
}

Answer 3

Realm allows it to use functions as parameter for the filtering. So this is my solution which im currently using.

The two filter functions:

func vegetarianFilter(_ meal: Meal) -> Bool {

        if showVegetarianOnly {
            if(meal.veg || meal.vegan){
                return true
            }

            return false
        }

        return true
}

func filterEmptyLines(_ line: Line) -> Bool {

       if(line.meals.filter(vegetarianFilter).count > 0){
           return true
       }

       return false
}

The functions filter all meals which are not vegetarian or vegan when the user has selected showVegetarianOnly = true. Also it filters all lines which than have no meal left (nothing is vegetarian or vegan).

Most important functions of the TableView:

    override func numberOfSections(in tableView: UITableView) -> Int {
            return canteenDay?.lines.filter(filterEmptyLines).count ?? 0
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
            return canteenDay?.lines.filter(filterEmptyLines)[section].meals.filter(vegetarianFilter).count ?? 0
    }

   override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        let cell = UITableViewCell()

        let meal = canteenDay!.lines.filter(filterEmptyLines)[indexPath.section].meals.filter(vegetarianFilter)[indexPath.row]

        cell.textLabel?.text = meal.meal

       return cell
    }