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Migrations - .NET SDK

A migration transforms an existing realm and its objects from its current Realm Schema version to a newer one. Application data models typically change over time to accommodate new requirements and features. Migrations give you the flexibility to automatically update your existing application data whenever a client application upgrades to a newer version.

A local migration is a migration for a realm that does not automatically Sync with another realm. Local migrations have access to the existing Realm Schema, version, and objects and define logic that incrementally updates the realm to its new schema version. To perform a local migration you must specify a new schema version that is higher than the current version and provide a migration function when you open the out-of-date realm.

MongoDB 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 that dynamically calculates a new property's value. Migrations do not allow you to directly rename a property. Instead, it treats a renamed property as a new property that you must set to the value of the old property before MongoDB Realm automatically deletes it.

MongoDB Realm handles migrations using the following framework:

Change Type
Description
Schema Version
MongoDB Realm ignores any schema version set by the client application.
Migration Function
MongoDB Realm throws an error if the realm configuration includes a local migration function.
Additions
MongoDB Realm automatically applies additive changes, such as a new class or class property.
Deletions

MongoDB Realm does not delete removed fields from the database, but ignores them in future read operations. MongoDB Realm includes deleted properties in new objects with a default value that depends on whether or not the property was required:

  • MongoDB Realm sets the value of optional deleted properties to null.
  • MongoDB Realm sets the value of required deleted properties to the empty value of the property type (e.g. 0 for numeric fields, "" for string properties, etc.).
Modifications

MongoDB Realm prevents modifications to an existing property other than delete operations. Modifications that synced realms do not support include:

  • Changing a property’s type but keeping the same name.
  • Changing an object type's primary key.
  • Changing a property from optional to required (or vice-versa).
  • A migration transforms an existing realm and its objects from its current schema version to a later one.
  • MongoDB Realm allows you to specify migration functions for local migrations, i.e. migrations where the realm is not synced with MongoDB Realm.
  • MongoDB Realm automatically handles synced migration, i.e. migrations where the realm is synced. MongoDB Realm does not allow migration functions for such migrations.
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