Drupal 7 Migrate: How do I migrate a mult-field key into a single taxonomy term
In your migration class, you can override the Migration::prepareRow() method to add a new name field to your rows:

  public function prepareRow($row) {
    $row-&gt;name = "{$row-&gt;element_id}-{$row-&gt;scale_id}-{$row-&gt;category}";
    return parent::prepareRow($row);
  }


And then simply map that field:

    $this-&gt;addFieldMapping('name', 'name');


But for simple concatenation, you can also use SelectQuery::addExpression() to add the CONCAT(...) to your SQL query:

    $name_alias = $query-&gt;addExpression(
      "CONCAT(element_id,'-',scale_id,'-',category)",
      'name'
    );


And then simply map the expression to the name field:

    $this-&gt;addFieldMapping('name', $name_alias);