What is the affection of insert order in SwiftData

Question

Created Jan ’24

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In SwiftData, what is the order of insertion for dependent and parent instances? In the snippet below, it works when the dependent instance is inserted before the parent. However, it fails when the order is reversed. Why is this the case?

//
//  DebugView.swift
//  gogodict
//
//  Created by djzhu on 2024/1/13.
//

import Foundation
import SwiftData
import SwiftUI

struct DebugView: View {
  @Query var parent: [ParentClass]
  @Query var dept: [DependentClass]
  var body: some View {
    Text("parent cnt:\(parent.count), dept cnt:\(dept.count)")
  }
}

@Model
class ParentClass {
  @Relationship(deleteRule: .cascade)
  var deps: DependentClass

  init(deps: DependentClass) {
    self.deps = deps
  }
}

@Model
class DependentClass {
  var id: String
  init(id: String = UUID().uuidString) {
    self.id = id
  }
}

#Preview {
  do {
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try ModelContainer(for: ParentClass.self, configurations: config)
    let context = ModelContext(container)

    var dept = DependentClass()
    var parentClass = ParentClass(deps: dept)

    /* ok to insert dept before parent. View shows: "parent cnt:1, dept cnt:1" */
    context.insert(dept) 
    context.insert(parentClass)

    /* ok to insert only parent, swiftData will insert dept too. View shows: "parent cnt:1, dept cnt:1" */
    // context.insert(parentClass) 

    /* failed to insert dept after parent, error list below */
    // context.insert(parentClass)
    // context.insert(dept) 

    return DebugView()
      .modelContainer(container)
  } catch {
    return Text("Failed to create container: \(error.localizedDescription)")
  }
}

The error log insert dept after parent:

Date/Time:           2024-01-13 14:10:10.3694 +0800
Launch Time:         2024-01-13 14:10:09.9932 +0800
OS Version:          macOS 14.1.1 (23B81)
Release Type:        User
Report Version:      104

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000001, 0x00000001929f3938
Termination Reason: SIGNAL 5 Trace/BPT trap: 5
Terminating Process: exc handler [51868]

Triggered by Thread:  0

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libswiftCore.dylib            	       0x1929f3938 _assertionFailure(_:_:file:line:flags:) + 248
1   SwiftData                     	       0x1c42fc454 0x1c42d1000 + 177236
2   SwiftData                     	       0x1c4315aac 0x1c42d1000 + 281260
3   SwiftData                     	       0x1c42fd4e0 0x1c42d1000 + 181472
4   DebugView.1.preview-thunk.dylib	       0x10161ae10 closure #1 in static $s39gogodict_PreviewReplacement_DebugView_133_20B25209EACDFA98A88DFB5B90B26E4CLl0B0fMf_15PreviewRegistryfMu_.makePreview() + 840 (@__swiftmacro_39gogodict_PreviewReplacement_DebugView_133_20B25209EACDFA98A88DFB5B90B26E4CLl0B0fMf_.swift:18)
5   PreviewsInjection             	       0x1d6aaa0ec 0x1d6a6f000 + 241900
6   PreviewsInjection             	       0x1d6aab060 0x1d6a6f000 + 245856
7   libswift_Concurrency.dylib    	       0x1e450b738 static MainActor.assumeIsolated<A>(_:file:line:) + 144
8   PreviewsInjection             	       0x1d6aa9e48 0x1d6a6f000 + 241224
9   PreviewsInjection             	       0x1d6aadfb8 0x1d6a6f000 + 257976
10  PreviewsInjection             	       0x1d6aaea88 0x1d6a6f000 + 260744
11  PreviewsInjection             	       0x1d6a9c89c 0x1d6a6f000 + 186524
12  PreviewsInjection             	       0x1d6a9f5c8 0x1d6a6f000 + 198088
13  PreviewsInjection             	       0x1d6a87c90 0x1d6a6f000 + 101520
14  PreviewsInjection             	       0x1d6a88128 0x1d6a6f000 + 102696
15  PreviewsInjection             	       0x1d6aa0fa0 0x1d6a6f000 + 204704
16  PreviewsInjection             	       0x1d6a777b8 0x1d6a6f000 + 34744
17  PreviewsInjection             	       0x1d6a77004 0x1d6a6f000 + 32772
18  PreviewsFoundation            	       0x1d69b4630 0x1d691c000 + 624176
19  libdispatch.dylib             	       0x18016b4f4 _dispatch_call_block_and_release + 24
20  libdispatch.dylib             	       0x18016cd3c _dispatch_client_callout + 16
21  libdispatch.dylib             	       0x18017bb24 _dispatch_main_queue_drain + 1272
22  libdispatch.dylib             	       0x18017b61c _dispatch_main_queue_callback_4CF + 40
23  CoreFoundation                	       0x1803f1a30 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
24  CoreFoundation                	       0x1803ec148 __CFRunLoopRun + 1936
25  CoreFoundation                	       0x1803eb5a4 CFRunLoopRunSpecific + 572
26  GraphicsServices              	       0x18e9fbae4 GSEventRunModal + 160
27  UIKitCore                     	       0x1852f02e4 -[UIApplication _run] + 868
28  UIKitCore                     	       0x1852f3f5c UIApplicationMain + 124
29  SwiftUI                       	       0x1c51fc1b0 0x1c4371000 + 15249840
30  SwiftUI                       	       0x1c51fc050 0x1c4371000 + 15249488
31  SwiftUI                       	       0x1c4f02fa4 0x1c4371000 + 12132260
32  gogodict                      	       0x1009f03b0 static GOGODictApp.$main() + 40
33  gogodict                      	       0x1009f0474 main + 12 (GOGODictApp.swift:12)
34  dyld_sim                      	       0x100bfd544 start_sim + 20
35  dyld                          	       0x100ce60e0 start + 2360

It seems perplexing to someone new like me. Is there any documentation or article that explains the underlying logic of the engine?

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