How to sub_process a non-associative array in migration.yml

Question

I know I can use sub_process to iterate over an associative array like

field_categories:
  plugin: sub_process
  source: remote_categories
  process:
    target_id:
      plugin: migration_lookup
      migration: my_existing_taxonomy_migration
      source: name_of_key

But I don't know how to iterate over a numeric / non-associative array. Assume the JSON data structure

{
  remote_categories: [ 'alpha', 'beta', 'gamma' ]
}

how can I pass the values as keys to migration_lookup? I.e. what would be name_of_key in the example above?

Answer 1

I've written a process plugin that transforms a sequential list into an array of associative sub-arrays, so sub_process and migration_lookup can use them.

namespace Drupal\my_module\Plugin\migrate\process;

use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;

/**
 * Transforms an array of values into an array of associative arrays.
 *
 * The inner array will be filled with one key. E.g. ['alpha', 'beta'] becomes
 * [[value => 'alpha'], [value => 'beta']]
 *
 * Use this plugin to preprocess a numeric/non-associative array for other
 * plugins that requires an associative array as input, such as the sub_process
 * plugin.
 *
 * Available configuration keys:
 * - source: Source property.
 * - keyname: name of the key to be used for the associative sub-arrays,
 *   defaults to 'value'.
 *
 * Example:
 *
 * @code
 * source:
 *   my_flat_array:
 *     - category1
 *     - category2
 * process:
 *   my_associative_array:
 *     plugin: deepen
 *     source: my_flat_array
 *   field_taxonomy_term:
 *     plugin: sub_process
 *     source: '@my_associative_array'
 *     process:
 *       target_id:
 *         plugin: migration_lookup
 *         migration: my_taxonomy_migration
 *         source: value
 * @endcode
 *
 * @see \Drupal\migrate\Plugin\MigrateProcessInterface
 *
 * @MigrateProcessPlugin(
 *   id = "deepen",
 *   handle_multiples = TRUE
 * )
 */
class Deepen extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $keyname = (is_string($this->configuration['keyname']) && $this->configuration['keyname'] != '') ? $this->configuration['keyname'] : 'value';

    if (is_array($value) || $value instanceof \Traversable) {
      $result = [];
      foreach ($value as $sub_value) {
        $result[] = [$keyname => $sub_value];
      }
      return $result;
    }
    else {
      throw new MigrateException(sprintf('%s is not traversable', var_export($value, TRUE)));
    }
  }

}

Answer 2

I have had success in two different ways. I found your question because I wanted "sub_process" to "key" my string value into an array, so it goes into a specific property on the field, not the default property.

  field_categories:
    -
      plugin: multiple_values
      source: remote_categories
    -
      plugin: migration_lookup
      migration: my_existing_taxonomy_migration
    -
      plugin: skip_on_empty
      method: process
      message: 'Migrate lookup failed, or no value passed!'
    -
      plugin: explode
      delimiter: 'StringThatShoudNotExistIntheValue'
    -
      plugin: sub_process
      process:
        target_id: '0'

Alternative method without sub_process (but will runs a process for each value).

  field_categories:
    -
      plugin: multiple_values
      source: remote_categories
    -
      plugin: migration_lookup
      migration: my_existing_taxonomy_migration
    -
      plugin: skip_on_empty
      method: process
      message: 'Migrate lookup failed, or no value passed!'

The explode() just casts the string of each array element into a array [0 => $value], then we use sub_process with target_id: 0 to key by the first/0-th element (which is $value).

The skip_on_empty is there for sanity check but is optional.