Change an Object Model - Node.js SDK

Schema Version

A schema version identifies the state of a realm schema at some point in time. Realm tracks the schema version of each realm and uses it to map the objects in each realm to the correct schema.

Schema versions are ascending integers that you can optionally include in the realm configuration when you open a realm. If a client application does not specify a version number when it opens a realm then the realm defaults to version 0.

Migrations

A migration is a function that updates a realm and any objects it contains from one schema version to a newer version. Migrations give you the flexibility to change your object schemas over time to accommodate new features and refactors.

Whenever you open an existing realm with a schema version greater than the realm's current version, Realm runs a migration function that you define. The function has access to the realm's version number and incrementally updates objects in the realm to conform to the new schema.

Realm automatically migrates certain changes, such as new and deleted properties, but does not automatically set values for new properties unless the updated object schema specifies a default value. You can define additional logic in the migration function to further customize property values.

Add a Property

To add a property to a schema, add the new property to the object's class and set a schemaVersion of the realm's configuration object.

const Person = {
  name: 'Person',
  properties: {
    firstName: 'string',
    lastName: 'string',
    age: 'int'
  }
}

const realm = await Realm.open({
    schema: [Person],
    schemaVersion: 2
  });

Delete a Property

To delete a property from a schema, remove the property from the object's class and set a schemaVersion of the realm's configuration object. Deleting a property will not impact existing objects.

const realm = await Realm.open({
    schema: [Dog],
    schemaVersion: 2
  });

Rename a Property

To rename an object property, change the property name in the object schema and then open the realm with an incremented schema version and a migration function that updates existing objects to use the new property name.

Migrations do not allow you to directly rename a property. Instead you can create a new property with the updated name, copy the value from the old property, and then delete the old property.

Realm.open({
  schema: [Person],
  schemaVersion: 2,
  onMigration: (oldRealm, newRealm) => {
    // only apply this change if upgrading to schemaVersion 2
    if (oldRealm.schemaVersion < 2) {
      const oldObjects = oldRealm.objects('Person');
      const newObjects = newRealm.objects('Person');
      // loop through all objects and set the fullName property in the new schema
      for (const objectIndex in oldObjects) {
        const oldObject = oldObjects[objectIndex];
        const newObject = newObjects[objectIndex];
        newObject.fullName = `${oldObject.firstName} ${oldObject.lastName}`;
      }
    }
  }
});

Modify a Property Type

To modify a property's type, set the property type of the field that you wish to modify to the new data type. Then, set a schemaVersion, and a migration callback function of the realm's configuration Object.

const realm = await Realm.open({
    schema: [Dog],
    schemaVersion: 2,
    onMigration: (oldRealm, newRealm) => {
        if(oldRealm.schemaVersion < 2){
            const oldObjects = oldRealm.objects('Dog');
            const newObjects = newRealm.objects('Dog');
            // loop through all objects and set the _id property in the new schema
            for (const objectIndex in oldObjects) {
              const oldObject = oldObjects[objectIndex];
              const newObject = newObjects[objectIndex];
              newObject._id = oldObject._id.toHexString();
            }
        }
    },
  });