Actually, it doesnt seem to be that hard .. in 2012. Yes, you have to keep track of source IDs versus import IDs, but the migrate module does this for you, in a neat little table. You can join that table in your source query, and update the node reference field with the nid of the .. referenced node. Ofcourse, the referenced nodes should have already been imported. If they werent, you can run an run an 'update' later and referenced nids get entered based on the latter imports too. In practice:
$query = Database::getConnection(
'default', 'mysourcedb'
)->select(
'mysourcetable','source'
)->fields('source', array(
'id',
'title',
'whatever'
'rel_rec_id'
)
);
$query->leftJoin('migrate_map_relimport','relmap','relmap.sourceid1=source.rel_rec_id');
$query->addField('relmap','destid1','rel_node_id');
The code above assumes you have a 'mysourcedb' with a 'mysourcetable' in it that refers to a 'rel_rec_id', and theres another import called RelImport that imports the rel table that rel_rec_id is refering to; it should have already run (or will run before you run an additional update). Do a migrate-status once you have the RelImport class to make sure the table exists.
To be able to make joins to the 'migrate_map_relimport' table, make sure map tables are written to the source database, not the drupal database. This is not always necessary, but here it is:
$this->map = new MigrateSQLMap(
$this->machineName,
array(
'id' => array(
'type' => 'int',
'unsigned' => true,
'not null' => true,
'alias' => 'source'
)
),
MigrateDestinationNode::getKeySchema(),
'mysourcedb' // connection to use to write map tables
);
and finally, assign the retrieved rel_node_id to your node reference:
$this->addFieldMapping( 'field_rel_node', 'rel_node_id' );
Yay, it is rocket science .. YMMV
*-pike
