SwiftData Query relationships not working

Overview

• I have 2 models: Deparment and Student
• Each Department can contain multiple students
• Each Student can only be in one Department
• I have DepartmentList, tapping on the department should take it to the StudentList which lists all students in the department

Problem

• When I use Query in StudentList to filter only students for a specific department id, no students are shown.

Questions:

What should I do to list the students in a department? (see complete code below).

let filter = #Predicate<Student> { student in
    student.department?.id == departmentID
}
let query = Query(filter: filter, sort: \.name)
_students = query

Complete code

App

@main
struct SchoolApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(for: [Department.self, Student.self])
    }
}

Department

import Foundation
import SwiftData

@Model
class Department {
    var id: UUID
    var name: String
    
    var students: [Student]

    init(
        id: UUID,
        name: String,
        students: [Student] = []
    ) {
        self.id = id
        self.name = name
        self.students = students
    }
}

Student

import Foundation
import SwiftData

@Model
class Student {
    var id: UUID
    var name: String
    
    @Relationship(inverse: \Department.students)
    var department: Department?
    
    init(
        id: UUID,
        name: String,
        department: Department? = nil
    ) {
        self.id = id
        self.name = name
        self.department = department
    }
}

ContentView

import SwiftUI

struct ContentView: View {
    
    @State private var selectedDepartment: Department?
    
    var body: some View {
        NavigationSplitView {
            DepartmentList(selectedDepartment: $selectedDepartment)
        } detail: {
            if let department = selectedDepartment {
                StudentList(department: department)
            } else {
                Text("no department selected")
            }
        }
        .task {
            printStoreFilePath()
        }
    }
    
    private func printStoreFilePath() {
        let urls = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask)
        if let path = urls.map({ $0.path(percentEncoded: false) }).first {
            print("Storage: \(path)")
        }
    }
}

DepartmentList

import SwiftUI
import SwiftData

struct DepartmentList: View {
    
    @Binding
    var selectedDepartment: Department?
    
    @Query(sort: \.name)
    private var departments: [Department]
    
    @Environment(\.modelContext)
    private var modelContext
    
    var body: some View {
        List(selection: $selectedDepartment) {
            ForEach(departments) { department in
                NavigationLink(value: department) {
                    Text(department.name)
                }
            }
        }
        .toolbar {
            ToolbarItem {
                Button {
                    addDepartment()
                } label: {
                    Label("Add", systemImage: "plus")
                }
            }
        }
    }
    
    private func addDepartment() {
        guard let index = (1000..<10000).randomElement() else {
            return
        }
        let department = Department(id: UUID(), name: "Department \(index)")
        modelContext.insert(department)
    }
}

StudentList

import SwiftUI
import SwiftData

struct StudentList: View {
    var department: Department
    
    @Query
    private var students: [Student]
    
    @Environment(\.modelContext)
    private var modelContext
    
    init(department: Department) {
        self.department = department
        let departmentID = department.id
        let filter = #Predicate<Student> { student in
            student.department?.id == departmentID
        }
        let query = Query(filter: filter, sort: \.name)
        _students = query
    }

    var body: some View {
        List {
            ForEach(students) { student in
                Text(student.name)
            }
        }
        .toolbar {
            ToolbarItem {
                Button {
                    addStudent()
                } label: {
                    Label("Add", systemImage: "plus")
                }
            }
        }
    }
    
    private func addStudent() {
        guard let index = (1000..<10000).randomElement() else {
            return
        }
        let student = Student(
            id: UUID(),
            name: "Student \(index)",
            department: department
        )
        modelContext.insert(student)
    }
}