Add the Authors migration

web/modules/custom/geo_upgrade/src/Plugin/migrate/source/Author.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\geo_upgrade\Plugin\migrate\source;
+
+use Drupal\migrate\Plugin\migrate\source\SqlBase;
+use Drupal\migrate\Row;
+
+/**
+ * Drupal 7 node source from database.
+ *
+ * @MigrateSource(
+ *   id = "geod7_author",
+ *   source_module = "geo_upgrade"
+ * )
+ */
+class Author extends SqlBase {
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareRow(Row $row) {
+    $field_author = $row->getSourceProperty('field_author_value');
+    // Make sure to all the author names are trimmed.
+    $row->setSourceProperty('field_author_value', trim($field_author));
+
+    return parent::prepareRow($row);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    $fields = [
+      'field_author_value' => $this->t('Author Name'),
+    ];
+    return $fields;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    $ids['field_author_value']['type'] = 'string';
+    $ids['field_author_value']['alias'] = 'fdfa';
+
+    return $ids;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    $select = $this->select('field_data_field_author', 'fdfa')
+      ->fields('fdfa', [
+        'field_author_value',
+      ])
+    ->groupBy('field_author_value');
+
+    return $select;
+  }
+
+}