Is it possible to create objects in onMigration

[PraneethGunas]

How frequently does the bug occur?

Always

Description

Trying to create objects in a new schema from the data in an old deleted schema.

onMigration: (oldRealm, newRealm) => {
    const oldObjects = oldRealm.objects(RealmSchema.OldSchemaName);
    oldObjects.forEach((obj) => {
      if (obj.arrayValue.length) {
        obj.arrayValue.forEach((arrItem) => {
          newRealm.write(() => {
            newRealm.create(RealmSchema.NewSchemaName, { key: oldObject.value, ..., ... });
          });
        });
      }
    });
  },

When I call newRealm.write(() => { newRealm.create(RealmSchema.NewSchemaName, {key, oldObject.value}, Realm.UpdateMode.All) }){{ it always throws }}Error: Exception in HostFunction: The Realm is already in a write transaction Assigning values to the new object is not a problem since it doesn't require a write operation but creating a new object from an old one seems like a problem. I think I'm missing something here. Please let me know if there's a workaround. The references doesn't show how to create a new object from old deleted objects.

Stacktrace & log output

Possible Unhandled Promise Rejection (id: 0):
Error: Exception in HostFunction: The Realm is already in a write transaction

Can you reproduce the bug?

Always

Reproduction Steps

• upgrade schema and version
• Realm fails to initialize when the onMigration callback runs at newRealm.write()

Version

11.10.2

What services are you using?

Local Database only

Are you using encryption?

Yes

Platform OS and version(s)

iOS/Android

Build environment

Which debugger for React Native: Flipper

Cocoapods version

No response

[kneth]

@PraneethGunas

When the migration function is called, the new Realm has already opened a transaction, and you can leave out newRealm.write().

If you have links to other objects - in particular embedded objects - you might have to transform the entire structure to plain JavaScript objects before inserting them into the new Realm.

[PraneethGunas]

@kneth Thanks a lot! removing newRealm.write() got rid of Error: Exception in HostFunction: The Realm is already in a write transaction. I now get Exception in HostFunction: Only Realm instances are supported. Maybe this has something with what you said...

If you have links to other objects - in particular embedded objects - you might have to transform the entire structure to plain JavaScript objects before inserting them into the new Realm.

newRealm.create(
            RealmSchema.NewSchemaName,
            {
              key1: oldObject.value1,
              key2: oldObject.value2,
              key3: {                          // object type with dynamic key
                [oldObject.value3]: {
                  key3_1: oldObject.value4,
                  key3_2: oldObject.value5,
                  key3_3: derivedValue(oldObject.value6),
                },
              },
            },
            Realm.UpdateMode.All
          );

The new schema looks like this (not embedded) but the values are derived from the oldObjects

const NewSchema: ObjectSchema = {
  name: RealmSchema.NewSchemaName,
  primaryKey: 'key_1',
  properties: {
    key_1: 'string',
    key_2: 'string',
    key_3: `{}`,
  },
};

[PraneethGunas]

Ah! I see there is something with the creation of the third key (key_3). Removing that helps me create the realm object without errors.

Edit: is it because Realm doesn't let you have dynamic keys for the dictionary-type objects? Can I not have a structure like {uniqueId1: object, uniqueId2: object} rather than [{id: uniqueId1, value: object}, {id: uniqueId2, value: object}]

[kneth]

is it because Realm doesn't let you have dynamic keys for the dictionary-type objects?

I expect it to.

As an alternative, can you create the object with key_1 and key_2 and add key_3 after? Something like:

const newObject = newRealm.create(RealmSchema.NewSchemaName, {
  key_1: oldObject.value1,
  key_2: oldObject.value2,
}, Realm.UpdateMode.All);
newObject.key_3.set({ 
  [oldObject.value3]:  {
    key3_1: oldObject.value4,
    key3_2: oldObject.value5,
    key3_3: derivedValue(oldObject.value6),
  }
});