MigrateSourceSQL breaks when SelectQueryInterface query contains union (and not wrapped in a subquery)

This may or may not be worth fixing -- but I figured it was worth noting here in case somebody has the same problem.

When you use a union on a query for a SQL migration, the following error is generated: "SQLSTATE[21000]: Cardinality violation: 1222 The used SELECT statements have a different number of columns". For example:

$query1 = db_select('node', 'n')->fields('n', array('nid', 'vid', 'title'));
$query2 = db_select('node', 'n')->fields('n', array('nid', 'vid', 'title'));
$query1->union($query2, "ALL");

$this->source = new MigrateSourceSQL($query1);

It would appear that columns are being added by the Migration Module to support what it is trying to do, but unfortunately drupal only adds the columns to the first statement in the union, and not the others. That leads to select statements with a different number of columns, and therefore a sql error.

The fix I came up with on my side was to wrap the unions up into a subquery, so added fields apply to the entire query unit:

$query1 = db_select('node', 'n')->fields('n', array('nid', 'vid', 'title'));
$query2 = db_select('node', 'n')->fields('n', array('nid', 'vid', 'title'));
$query1->union($query2, "ALL");

$queryx = db_select($query1,'x'); // wraps the 2 unions into 1 subselected table 'x'

$this->source = new MigrateSourceSQL($queryx);

Easy enough, but this probably should be done within the migrate module itself.