Drupal 8 Migration with Multiple Paragraph References

September 27, 2018
As of Fall 2018 and the release of Drupal 8.6, the Migrate module in core is finally stabilizing! Hopefully Migrate documentation will continue to solidify, but there are plenty of gaps to fill. Recently, I ran into an issue migrating Paragraph Entities (Entity Reference Revisions) that had a few open core bugs and ended up being really simple to solve within prepareRow in the source plugin.

Setup

In my destination D8 site, I had a content type with a Paragraph reference field. Each node contained one or more Paragraph entities. This was reflected by having a Node migration with a dependency on a Paragraph Entity migration. With single value entity references, the migration_lookup plugin makes it really easy lookup up entity reference identifiers that were previously imported. As of September 2018, there is an open core issue to allow multiple values with migration_lookup. Migration lookup uses the migration map table created in the database to connect previously migrated data to Drupal data. The example below lookups up the taxonomy term ID based on the source reference (topic_area_id) from a previously ran migration. Note: You will need to add a migration dependency to your migration yml file to make sure migrations are run in the correct order. 
 field_resource_category:
   plugin: migration_lookup
   migration: nceo_migrate_resource_category
   no_stub: true
   source: topic_area_id

Solution

Without using a Drupal Core patch, we need a way to do a migration_lookup in a more manual way. Thankfully prepareRow in your Migrate source plugin makes this pretty easy. Note: This is not a complete Migrate source plugin. All the methods are there, but I’m focussing on the prepareRow method for this post. The most important part of the code is manually querying the Migrate map database table created in the Paragraph Entity migration. 
<?php
 
namespace Drupal\your_module\Plugin\migrate\source;
 
use Drupal\Core\Database\Database;
use Drupal\migrate\Plugin\migrate\source\SqlBase;
use Drupal\migrate\Row;
 
/**
* Source plugin for Sample migration.
*
* @MigrateSource(
*   id = "sample"
* )
*/
class Sample extends SqlBase {
 
 /**
  * {@inheritdoc}
  */
 public function query() {
   // Query source data.
 }
 
 /**
  * {@inheritdoc}
  */
 public function fields() {
   // Add source fields.
 }
 
 /**
  * {@inheritdoc}
  */
 public function getIds() {
   return [
     'item_id' => [
       'type' => 'integer',
       'alias' => 'item_id',
     ],
   ];
 }
 
 /**
  * {@inheritdoc}
  */
 public function prepareRow(Row $row) {
   // In migrate source plugins, the migrate database is easy.
   // Example: $this->select('your_table').
   // Getting to the Drupal 8 db requires a little more code.
   $drupalDb = Database::getConnection('default', 'default');
 
   $paragraphs = [];
   $results = $drupalDb->select('your_migrate_map_table', 'yt')
     ->fields('yt', ['destid1', 'destid2'])
     ->condition('yt.sourceid2', $row->getSourceProperty('item_id'), '=')
     ->execute()
     ->fetchAll();
   if (!empty($results)) {
     foreach ($results as $result) {
       // destid1 in the map table is the nid.
       // destid2 in the map table is the entity revision id.
       $paragraphs[] = [
         'target_id' => $result->destid1,
         'target_revision_id' => $result->destid2,
       ];
     }
   }
 
   // Set a source property that can be referenced in yml.
  // Source properties can be named however you like.
   $row->setSourceProperty('prepare_multiple_paragraphs', $paragraphs);
 
   return parent::prepareRow($row);
 }
 
}
In your migration yml file, you can reference the prepare_multiple_paragraphs that was created in the migrate source plugin like this: 
id: sample
label: 'Sample'
source:
 plugin: sample
process:
 type:
   plugin: default_value
   default_value: your_content_type
 field_paragraph_field:
   source: prepare_multiple_paragraphs
   plugin: sub_process
   process:
     target_id: target_id
     target_revision_id: target_revision_id
Sub_process was formally the iterator plugin and allows you to loop over items. This will properly create references to multiple Paragraph Entities. It will be nice when the migration_lookup plugin can properly handle this use case, but it’s a good thing to understand how prepareRow can provide flexibility.
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