Added Deepen.

src/Plugin/migrate/process/Deepen.php

+<?php
+
+namespace Drupal\migrate_process_array\Plugin\migrate\process;
+
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Wraps each item in an array with an array.
+ *
+ * Deepen is the opposite of flatten. After processing a field, particularly
+ * when merging fields together, you may get something like the following:
+ *
+ * @code
+ * array(
+ *   0 => '123',
+ *   1 => '456',
+ *   2 => '789',
+ * );
+ * @endcode
+ *
+ * Each value is intended as a single field value in a multi-value field, rather
+ * than as partials of one field. What we need is to reformat the array as such:
+ *
+ * @code
+ * array(
+ *   0 => array(
+ *     0 => '123',
+ *   ),
+ *   1 => array(
+ *     0 => '456',
+ *   ),
+ *   2 => array(
+ *     0 => '789',
+ *   ),
+ * );
+ * @endcode
+ *
+ * Sometimes we need to set the key for each "deepened" value. We can do so with
+ * the 'key' item:
+ *
+ * @code
+ * my_field:
+ *   source: my_source
+ *   plugin: deepen
+ *   key: 'target_id'
+ * @endcode
+ *
+ * This would result in output:
+ *
+ * @code
+ * array(
+ *   0 => array(
+ *     'target_id' => '123',
+ *   ),
+ *   1 => array(
+ *     'target_id' => '456',
+ *   ),
+ *   2 => array(
+ *     'target_id' => '789',
+ *   ),
+ * );
+ * @endcode
+ *
+ * Then we can do run the result through sub_process:
+ *
+ * @code
+ * field_media:
+ *   -
+ *     source: the_source
+ *     plugin: deepen
+ *     key: 'target_id'
+ *   -
+ *     plugin: sub_process
+ *     process:
+ *       target_id: target_id
+ * @endcode
+ *
+ * @MigrateProcessPlugin(
+ *   id = "deepen"
+ * )
+ */
+class Deepen extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    // Only process non-empty values.
+    if (empty($value)) {
+      return NULL;
+    }
+
+    // The input must be an array.
+    if (!is_array($value)) {
+      $value = [$value];
+    }
+
+    $out = [];
+    foreach ($value as $item) {
+      if (empty($this->configuration['key'])) {
+        $out[] = [$item];
+      }
+      else {
+        $out[] = [$this->configuration['key'] => $item];
+      }
+    }
+
+    return $out;
+  }
+}

src/Plugin/migrate/process/ExtractSingle.php

@@ -9,6 +9,7 @@ use Drupal\migrate\Plugin\migrate\process\Extract;
  *
  * Normally, a field value is like this:
  *
+ * @code
  * array(
  *    0 => array(
  *      'my_key' => 'The value we want',
@@ -17,24 +18,29 @@ use Drupal\migrate\Plugin\migrate\process\Extract;
  *      'my_key' => 'Another value we want',
  *    ),
  * )
+ * @endcode
  *
  * With core's extract, we have can only get the first as we have to specify
  * the field delta key.
  *
+ * @code
  * my_field:
  *   source: some_field
  *   plugin: extract
  *   index:
  *     - 0
  *     - my_key
+ * @endcode
  *
  * With this plugin, we can get the value within each field:
  *
+ * @code
  * my_field:
  *   source: some_field
  *   plugin: extract_single
  *   index:
  *     - my_key
+ * @endcode
  *
  * @MigrateProcessPlugin(
  *   id = "extract_single",