How to migrate products display nodes with multiple product references? I have a site being migrated from Drupal 6 to 7. The old site had a home grown system for adding payments to signups for event nodes. We plan to use commerce module to selling the registrations for events going forward.

In the new site, we are planning to import the preexisting SKUs from the old site with feeds but I want to include the product references with the event migration we have written. This migration is already working and just needs to have the product references added to it.

My question is about how to prepare the source data to be mapped when it looks something like this:

nid | sku
---------
1   | a
1   | b
1   | c
2   | a
2   | d


When it seems like it would be better if the data was like this:

nid | sku
---------
1   | a,b,c
2   | a,d


I think if I had the like the second table, I could join the source table

// Join payment_options_skus table
$query-&gt;join('payment_options_skus', 'pos', 'pos.nid = n.nid');
$query-&gt;fields('pos', array('sku'));


and then map like so: 

$this-&gt;addFieldMapping('field_registration', 'sku')
     -&gt;separator(',');


Is there a better way to do it using the existing data in the first example table? I would like to limit the number of manual massaging steps needed. Thanks.
I'm in a similar boat, but doing an import for a content type. If data is prepared with multi-value fields as in your second table, I can import the data using Feeds and the plugin 'Feeds Tamper'. So I'm also looking at repeatable and efficient ways to arrange data with multi-fields in CSV format. Saw a post about using Drupal 7 'Feeds Tamper PHP' to do create the multi-valued fields here, https://www.reddit.com/r/drupal/comments/2tumv9/drupal_7x_feeds_tamper_column_values_to_single/co2hesj. &lt;-- Untested, just a reference. HTH.