Issue #2747939 by generalredneck: Allow Merging two arrays together in migration

src/Plugin/migrate/process/Merge.php

+<?php
+
+namespace Drupal\migrate_plus\Plugin\migrate\process;
+
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Row;
+
+/**
+ * This plugin merges arrays together.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "merge"
+ * )
+ *
+ * Use to merge several fields into one. In the following example, imagine a D7
+ * node with a field_collections field and an image field that migrations were
+ * written for to make paragraph entities in D8. We would like to add those
+ * paragraph entities to the 'paragraphs_field'. Consider the following:
+ *
+ *  source:
+ *    plugin: d7_node
+ *  process:
+ *    temp_body:
+ *      plugin: iterator
+ *      source: field_section
+ *      process:
+ *        target_id:
+ *          plugin: migration
+ *          migration: field_collection_field_section_to_paragraph
+ *          source: value
+ *    temp_images:
+ *      plugin: iterator
+ *      source: field_image
+ *      process
+ *        target_id:
+ *          plugin: migration
+ *          migration: image_entities_to_paragraph
+ *          source: fid
+ *    paragraphs_field:
+ *      plugin: merge
+ *      source:
+ *        - @temp_body
+ *        - @temp_images
+ *  destination:
+ *    plugin: 'entity:node'
+ */
+class Merge extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!is_array($value)) {
+      throw new MigrateException('Input should be an array.');
+    }
+    $new_value = [];
+    foreach($value as $item) {
+      if (!is_array($item)) {
+        throw new MigrateException('One of the items is not an array that can be merged.');
+      }
+      $new_value = array_merge($new_value, $item);
+    }
+    return $new_value;
+  }
+
+}