Change an Object Model - Node.js SDK
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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.
Important
Increment Versions Monotonically
Migrations must update a realm to a higher schema version. Realm will throw an error if a client application opens a realm with a schema version that is lower than the realm's current version or if the specified schema version is the same as the realm's current version but includes different object schemas.
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.
Tip
Bypass Migration During Development
When developing or debugging your application, you may prefer to delete
the realm instead of migrating it. Use the
deleteRealmIfMigrationNeeded flag to delete the database
automatically when a schema mismatch would require a migration.
Never release an app to production with this flag set to true.
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.
Note
Modify Schema Properties of a Synced Realm
The following page demonstrates how to modify schema properties of a local realm. Learn how to modify schema properties of a synced realm.
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.
Example
A realm using schema version 1 has a Person object type with a
firstName, and lastName property. The developer decides to add an
age property to the Person class.
To migrate the realm to conform to the updated Person schema, the
developer sets the realm's schema version to
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.
Example
A realm using schema version 1 has a Dog object type with a
weight property. The developer decides to remove the property from the
schema.
To migrate the realm to conform to the updated Dog schema, the
developer sets the realm's schema version to
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.
Example
A realm using schema version 1 has a Person object type. The
original schema had a firstName and lastName field. The developer
later decides that the Person class should use a combined fullName
field and removes the separate firstName and lastName fields.
To migrate the realm to conform to the updated Person schema, the
developer sets the realm's schema version to
2 and defines a migration function to set the value of fullName based
on the existing firstName and lastName properties.
Important
Synced Realms
Synced realms only support non-breaking - also called additive - changes to ensure that older clients can sync with newer clients. Because full renames require you to delete the old property, you cannot rename a synchronized property without requiring a client reset. Instead, consider adding the renamed property without deleting the old property. Alternately, use mapTo to store data using the existing internal name, but let your code use a different name.
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.
Note
Synchronized realms only support non-breaking changes, to ensure that older clients can sync with newer clients. This means that synchronized realms do not support modifying the type of a property of a schema.
Example
A realm using schema version 1 has a Dog object type. The
original schema had an _id with a property type of Object ID. The
developer later decides that the Dog class's _id field should be of
type string, and updates the schema.
To migrate the realm to conform to the updated Dog schema, the
developer sets the realm's schema version to
2 and defines a migration function to convert the Object ID type to a
string type.