Data Loss in Nested Objects When Using Spread Operator for Update in Realm JS about realm-js HOT 6 CLOSED

➤ PM Bot commented:

Jira ticket: RJS-2847

from realm-js.

@tharwi I am not able to reproduce. How does your code differs from #6774?

from realm-js.

Hi @kneth, thanks for the reply.

Can you change the below line

      const updatedProfile = { ...currentProfile, lastAccessDate: new Date() };

to this

const profile = realmInstance.objects('Profile')[0];
const updatedProfile = {...profile, lastAccessDate: new Date()};

from realm-js.

@tharwi, the issue you're seeing is more related to what you're assigning rather than the spread operation itself. When you're using the spread operator it performs a shallow copy, so the trackingDataList on your object will still be the Realm.List from your currentProfile.

This means that when you pass that Realm.List to realm.create(.., .., UpdateMode.Modified), it will basically perform a self-assignment (e.g. object.list = object.list). We do not cache our collections, so even though you're using UpdateMode.Modified, we currently do not know if it's actually the same list being assigned in this case, and we need to clear the underlying collection being assigned to before adding the items of the RHS list (which is why the list becomes empty here).

This is of course something we'd like to fix as soon as possible and the initial work can be tracked here.

As an example, let's say you want to update the valueToUpdate property below:

class ListItem extends Realm.Object {
  value!: number;

  static schema: ObjectSchema = {
    name: "ListItem",
    properties: {
      value: "int",
    },
  };
}

class ObjectWithList extends Realm.Object {
  _id!: BSON.ObjectId;
  list!: Realm.List<ListItem>;
  valueToUpdate!: string;

  static schema: ObjectSchema = {
    name: "ObjectWithList",
    primaryKey: "_id",
    properties: {
      _id: "objectId",
      list: "ListItem[]",
      valueToUpdate: "string",
    },
  };
}

const realm = new Realm({ schema: [ObjectWithList, ListItem] });

const _id = new BSON.ObjectId();
const object = realm.write(() => {
  return realm.create(ObjectWithList, { _id, list: [{ value: 1 }], valueToUpdate: "original" });
});

expect(object.list.length).equals(1);

const objectShallowCopy = { ...object };

// Since it's a shallow copy, the list is still the Realm List.
expect(objectShallowCopy.list).to.be.instanceOf(Realm.List);

realm.write(() => {
  // Unfortunately, passing in the same Realm List again basically
  // performs a self-assignment, clearing the list.
  return realm.create(ObjectWithList, objectShallowCopy, UpdateMode.Modified);
});

// 💥 This will fail.
expect(object.list.length).equals(1);

Workaround:

From the code example you provided, it looks like you only want to update your lastAccessDate field. You can instead pass only the fields to be updated to realm.create() when using UpdateMode.Modified, rather than passing the spread.

realm.write(() => {
-  return realm.create(ObjectWithList, objectShallowCopy, UpdateMode.Modified);
+  return realm.create(ObjectWithList, { _id, valueToUpdate: "updated" }, UpdateMode.Modified);
});

Or skip realm.create() and update the field on the object directly:

 realm.write(() => {
-  return realm.create(ObjectWithList, objectShallowCopy, UpdateMode.Modified);
+  object.valueToUpdate = "updated";
});

from realm-js.

Hi @elle-j , Thanks for the update. Will use one of the workaround for now.

from realm-js.

Closing since this is tracked in realm/realm-core#7422.

from realm-js.