MigrationSourceManager checks that CollectionMetadata is set properly – collVersion, keyPattern. https://github.com/mongodb/mongo/blob/cdc7af4c6d453b8c1ce2319d0cd3b50074609d87/src/mongo/db/s/migration_source_manager.cpp#L145-L149 This should be something that CollectionMetadata assures, rather than specifically checked by CollectionMetadata users.