Source databases with fields/columns named using spaces or reserved words

Drupal's database abstraction layer cannot directly access columns named using spaces or reserved words: #1426084: Provide backtick escaping for MySQL in DB abstraction layer.

You can fix your source database column names or use the following workaround to specify the column name specifically. In this example, fulltext is a SQL reserved word.

$query = Database::getConnection('default', 'joomla_migrate')
  ->select('jos_content', 'jc')
  ->fields('jc', array('id', 'title', 'introtext', 'created', 'modified'));
$query->addExpression('`fulltext`', 'full');