Configure relationships with SwiftData in Preview

Question

We are developing an iOS app using SwiftData. We have Bucket and BucketItem models where a Bucket can have many BucketItem, so there is a relationship between Bucket and BucketItem. Below is a simplified version of the models.

import Foundation
import SwiftData

@Model
class Bucket {
  var name: String
  var shared: Bool
  
  @Relationship(deleteRule: .cascade, inverse: \BucketItem.bucket) var items = [BucketItem]()
  
  init(name: String, shared: Bool) {
    self.name = name
    self.shared = shared
  }
}

---

import Foundation
import SwiftData

@Model
class BucketItem {
  var content: String
  var bucket: Bucket
  
  init(content: String, bucket: Bucket) {
    self.content = content
    self.bucket = bucket
  }
}

We show a list of Bucket with a count of how many BucketItem it has. This is working in the app on the device and simulator, however the relationship is not working on the Preview. Below is a simplified version of this component with the Preview configuration

import SwiftUI
import SwiftData

struct BucketsData: View {
 var bucket: Bucket //this is provided by a parent component that uses @Query to fetch the Buckets
  
  var body: some View {
    List {
      Section(bucket.name) {
        Text("Items: \(bucket.items.count)")
      }
      
    }
  }
}

#Preview {
  let config = ModelConfiguration(isStoredInMemoryOnly: true)
  let container = try! ModelContainer(for: Bucket.self, configurations: config)
  
  let testBucket = Bucket(name: "Test Bucket", shared: true)
  container.mainContext.insert(testBucket)
  
  let testItem = BucketItem(content: "Test Item", bucket: testBucket)
  container.mainContext.insert(testItem)
  
  return BucketsData(bucket: testBucket)
    .modelContainer(container)
}

The list show the "Test Bucket" with a count of 0 items. It appears that for some reason the items array is not getting populated with the related BucketItem. How can we fix this?