SQLSTATE[42S02]: Base table or view not found: 1146 Table 'migration_db.taxonomy_index' doesn't exist:    [error]
SELECT 1 AS expression
FROM
{taxonomy_index} taxonomy_index
WHERE ( (nid = :db_condition_placeholder_0) AND (tid = :db_condition_placeholder_1) AND (status =
:db_condition_placeholder_2) ); Array
(
    [:db_condition_placeholder_0] => 2290
    [:db_condition_placeholder_1] => 4
    [:db_condition_placeholder_2] => 1
)
 (/home/vagrant/docroot/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php:770)

...
process:
  field_section: Section
...

class MyNodeMigration extends SqlBase {
  public function query() {
    return $this->select('TableA', 'ta')
      ->fields('ta', ['Id', 'Body', 'OriginalId']);
  }
 ...
  public function prepareRow(Row $row) {
    $row->setSourceProperty('Section', $this->convertToTermId($row->getSourceProperty('OriginalId'));
  }

  public function convertToTermId($original_id) {
    return $this->getDatabase()->select('TableB', 'tb')
      ->fields('tb', ['DrupalTermId'])
      ->condition('OldId', $original_id)
      ->execute()
      ->fetchField();
  }
}