fix: core data crash with inverse attribute

[rnr]

This PR fixes crash:

[volodymyr-chekyrta]

@IvanStepanok will review this, because deletionRule=“Cascade” was done intentionally to fix a different crash case.

[IvanStepanok]

Hi @rnr, great that you hit on this topic. This problem sometimes haunts the project and I think I have found a solution. We used cascading deletion of the database so that no garbage is left behind when deleting, but we didn't take into account that we need to clearly define the relationships between the database objects. I set up the relationships and it works great! Now when I delete a database due to a conflict, the application does not crash. To test it, I tried to write a function that I run right at the start of the application when creating a context:

      private func createContext() -> NSManagedObjectContext {
        let context = persistentContainer.newBackgroundContext()
        context.automaticallyMergesChangesFromParent = true
        createErrorCausingEntities(context) // <-- CALL HERE
        return context
    }

   func createErrorCausingEntities(_ context: NSManagedObjectContext) {
           
           context.perform {
               context.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
               // Create CDPrimaryCourse
               let primaryCourse = CDPrimaryCourse(context: context)
               primaryCourse.name = "Test Course"
               
               // Create CDAssignment without setting up the inverse relationship
               let assignment = CDAssignment(context: context)
               assignment.title = "Test Assignment"
               
               // Attempt to add the assignment to futureAssignments without proper setup
               primaryCourse.futureAssignments = NSSet(array: [assignment])
               
               do {
                   try context.save()
               } catch {
                   print("Error saving context: \(error)")
               }
           }
       }

There is screenshots with all relationships settings. You can try it:

I hope this solution will fix this crash🙏

[rnr]

Thank you @IvanStepanok !
I tried this and looks good. Lets wait opinion from @forgotvas (he had fixed this crash)

[forgotvas]

hi @IvanStepanok, yes main point is to add relationship between CDPrimaryCourse and CDAssignment, Nulify rule was committed by my fault, so our fix should work fine with Cascade rule too. Like you suggested in your snapshot, but you did use new fields for relationship.

Can you provide example what crash fixed by Cascade rule?

[IvanStepanok]

Hi @forgotvas

We added a cascading deletion rule to fix an issue where reloading CDPrimaryCourse did not overwrite nested objects properly. By using "Cascade," related entities (like CDAssignment) are now deleted alongside their parent, ensuring that when reloading, new data can be saved without conflicts from residual old data.

[forgotvas]

Hi @forgotvas

We added a cascading deletion rule to fix an issue where reloading CDPrimaryCourse did not overwrite nested objects properly. By using "Cascade," related entities (like CDAssignment) are now deleted alongside their parent, ensuring that when reloading, new data can be saved without conflicts from residual old data.

do you have steps to reproduce that crash, want to try to repeat it in my branch with my fix.

[IvanStepanok]

do you have steps to reproduce that crash, want to try to repeat it in my branch with my fix.

As I remember, you need a really large course to reproduce this error. On the first attempt, caching works fine, but on the second time, there's a cascade of core data errors. Nothing special, the app still works, but the cache is not updated.

[rnr]

So @IvanStepanok @forgotvas
As I see there is no difference between your fixes - just Ivan creates separate Relationships to relates entities but Vadim adds inverseEntity field. Is this right? Then lets select one of these approaches and move forward. WDYT?

[volodymyr-chekyrta]

Please resolve git conflicts

[rnr]

Conflict is resolved
Also changed deletionRules to 'Cascade' as @IvanStepanok suggested

[volodymyr-chekyrta]

LGTM