Each external object has an External ID standard field. Its values uniquely identify each external object record in your organization. When the external object is the parent in an external lookup relationship, the External ID standard field is used to identify the child records.
override global List<DataSource.Table> sync() { List<DataSource.Table> tables = new List<DataSource.Table>(); List<DataSource.Column> columns; columns = new List<DataSource.Column>(); columns.add(DataSource.Column.text('title', 255)); columns.add(DataSource.Column.text('description',255)); columns.add(DataSource.Column.text('createdDate',255)); columns.add(DataSource.Column.text('modifiedDate',255)); columns.add(DataSource.Column.url('selfLink')); columns.add(DataSource.Column.url('DisplayUrl')); columns.add(DataSource.Column.text('ExternalId',255)); tables.add(DataSource.Table.get('googleDrive','title', columns)); return tables; }