Migrate Source CSV is currently the source plugin of choice for doing a CSV-to-Drupal migration with the Migrate API in Drupal 8. In this post I will demonstrate how to manipulate the CSV data in realtime during the migrate:import operation. You can think of this as the equivalent to prepareRow() that you have seen elsewhere, like my blog post Extending the Migrate Plus JSON Parser in Drupal 8.

Please make sure you have a working migration before you begin; it’ll make things easier to troubleshoot if you know you had a good starting point.