deferred

Foreign keys are checked right before changes are committed on disk, unless the DatabaseMigrator is the result of disablingDeferredForeignKeyChecks.

In this case, you can perform your own deferred foreign key checks with checkForeignKeys(in:in:) or checkForeignKeys:

migrator = migrator.disablingDeferredForeignKeyChecks()
migrator.registerMigration("Partially checked migration") { db in
    ...

    // Throws an error and stops migrations if there exists a
    // foreign key violation in the 'book' table.
    try db.checkForeignKeys(in: "book")
}