Parent Child delete not working with ConcurrentModificationException - tried diff ways

[vvictor10]

Simple relationship - Car has 1 or more tires. Using version 0.79.1. Due to the absence of cascade delete support, I am trying to delete the children before deleting the parent object(as part of the result collection).

In all cases, I get the same exception. Can you guys tell me how I am supposed to handle this? Cascade delete would be perfect, I just would love to save RealmResult.clear(). Without that, how do I delete the parent and the children. Thanks in advance.

java.util.ConcurrentModificationException: No outside changes to a Realm is allowed while iterating a RealmResults. Use iterators methods instead.
        at io.realm.RealmResults.assertRealmIsStable(RealmResults.java:507)
        at io.realm.RealmResults.access$200(RealmResults.java:55)
        at io.realm.RealmResults$RealmResultsIterator.hasNext(RealmResults.java:524)

Option 1:

    realm.beginTransaction();
    RealmResults<RealmCar> cars = realm.where(RealmCar.class).findAll();
    // Cascade delete is not available as of version 0.79.1.
    for(RealmCar car : cars){
        car.getTires().clear();
    }
    int result = cars.size();
    cars.clear();
    realm.commitTransaction();

Option 2:

    realm.beginTransaction();
    RealmResults<RealmCar> cars = realm.where(RealmCar.class).findAll();
    // Cascade delete is not available as of version 0.79.1.
    for(RealmCar car : cars){
        Iterator<RealmTire> tireIterator = car.getTires().iterator();
        while(tireIterator.hasNext()){
            tireIterator.next().removeFromRealm();
        }
    }
    int result = cars.size();
    cars.clear();
    realm.commitTransaction();

Option 3:

    realm.beginTransaction();
    RealmResults<RealmCar> cars = realm.where(RealmCar.class).findAll();
    // Cascade delete is not available as of version 0.79.1.
    Iterator<RealmCar> carIterator = cars.iterator();
    while(carIterator.hasNext()){
        Iterator<RealmTire> tireIterator = carIterator.next().getTires().iterator();
        while(tireIterator.hasNext()){
            tireIterator.next().removeFromRealm();
        }
    }
    int result = cars.size();
    cars.clear();
    realm.commitTransaction();
    realm.close();

[emanuelez]

Hello,
that is an area of the API we need to improve and we are working on it.
While we fix this you could try something like this:

for (int i = cars.size() - 1; i >= 0; i--) {
   Car car = cars.get(i);
   List<Tire> tires = car.getTires();
   for (int j = tires.size() - 1; j >= 0; j--) {
      tires.get(j).removeFromRealm();
   }
}

[vvictor10]

First of all, thanks a lot for the almost immediate response.
That worked. Any idea when we can get the cascade delete feature?

[emanuelez]

That's a controversial issue since there is no right way to handle cascading deletes.
For some use cases you want it to be aggressive while in others you want it to be conservative, so it's a hard balance.

For example, imagine you have a Developer class and an Issue class. A developer can be assigned to several issues.
If that developer leaves the company you only want to delete him and not the issues he's assigned to.

So we need to be able to support all use cases and the current state, while vastly sub-optimal, is at least flexible enough to handle all the use-cases we came across so far.

[vvictor10]

@emanuelez Agreed. No doubt, but it should be configurable through an annotation at least? I am thinking that there would be more parent/child relationship where the Child makes no sense without a parent than the other way(for e.g. like the one you are mentioning). In all those cases, it would be super useful to just blow the whole thing away in one method call.

[mmvicsonvictor]

The suggested alternative is a pretty slow operation as you can tell. I have a car which has 4 tires. To delete 10000 cars(4 tires each) takes about 13 seconds, which is unacceptable even on a background operation. I can see several applications performing background syncs where wiping data and recreating them is pretty common.

Before you guys address the cascade issue, can we have an interim solution to delete all the children in one shot? Something like a removeAllFromRealm() on a list.

This is my current code, is there a better way to do it?

    Realm realm = realmInstanceFactory.getInstance();
    realm.beginTransaction();
    RealmResults<RealmCar> cars = realm.where(RealmCar.class).findAll();
    int result = cars.size();

    if(result != 0){
        for (int i = cars.size() - 1; i >= 0; i--) {
            RealmCar car = cars.get(i);
            List<RealmTire> tires = car.getTires();
            for (int j = tires.size() - 1; j >= 0; j--) {
                tires.get(j).removeFromRealm();
            }
        }
        cars.clear();
    }
    realm.commitTransaction();
    realm.close();

[mmvicsonvictor]

@cmelchior Has there been any movement on this issue? Thanks.

[osayilgan]

@emanuelez is there any progress on this issue ? It's a sort of critical issue for my case.

[lukcz]

@emanuelez any timeframe for this? As @osayilgan already said this is kind of critical functionality (also for me)

[mustafa01ali]

Bump. Any update on this?

[emanuelez]

Yes, we now have the functionality we needed from the core so we can now implement it in the binding

[aercanozcan]

do we have it now?

[emanuelez]

This issue can be splitted in two major parts:

• Cascading deletes: this is being worked on by the core tram
• Being able to delete items while iterating: this high in the to-do list of the java team

[cmelchior]

As @emanuelez says, this issue is already covered by #1104 and #640. So i'll close this as an duplicate.