drupal8-migrations/docroot/modules/custom/sample_migration/sample_migration.module

<?php

/**
 * @file
 * Contains sample_migration.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function sample_migration_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the sample_migration module.
    case 'help.page.sample_migration':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Sample Migrations') . '</p>';
      return $output;

    default:
  }
}

function sample_migration_title_body_paragraphs(&$context) {
  $entity_type_manager = \Drupal::service('entity_type.manager');

  /* @var NodeStorage $node_storage */
  $node_storage = $entity_type_manager->getStorage('node');
  $media_storage = $entity_type_manager->getStorage('media');
  $file_storage = $entity_type_manager->getStorage('file');

  if (empty($context['sandbox'])) {
    $context['sandbox'] = [];
  }

  if (!isset($context['sandbox']['progress'])) {
    $context['sandbox']['progress'] = 0;
    $context['sandbox']['current_index'] = 0;
    $all_arm_pages = $node_storage->loadByProperties([
      'type' => 'arm_page'
    ]);
    $context['sandbox']['arm_content'] = array_values($all_arm_pages);
    $context['sandbox']['arm_map'] = _get_arm_map();
    $context['sandbox']['file_map'] = _get_file_map();
    $context['sandbox']['d7_files'] = _get_d7_arm_files();

    $context['sandbox']['index'] = array_values($context['sandbox']['arm_content']);
    $context['sandbox']['max'] = count($context['sandbox']['arm_content']);
  }

  $limit = 10;
  $indexes = range($context['sandbox']['current_index'], $context['sandbox']['current_index'] + $limit);

  foreach ($indexes as $index) {
    $node = $context['sandbox']['arm_content'][$index];

    if (!is_null($node)) {
      $context['results'][] = $node->title->value;
      $d7nid = $context['sandbox']['arm_map'][$node->id()];
      $d7file = $context['sandbox']['d7_files'][$d7nid->sourceid1];

      if ($d7file) {
        $d8file = $context['sandbox']['file_map'][$d7file->field_armpage_pdf_fid];
        $current_file = $file_storage->load($d8file->destid1);

        if ($d8file) {
          $node->set('field_armpage_pdf', $current_file->id());
          $node->save();
        }
      }
    }

    $context['sandbox']['progress']++;
    $context['sandbox']['current_index'] = $index;
    $context['message'] = t('Now processing Reviews');
  }

  // Update batch on our progress.
  if ($context['sandbox']['progress'] != $context['sandbox']['max']) {
    $context['finished'] = $context['sandbox']['progress'] / $context['sandbox']['max'];
  }
  else {
    $context['sandbox']['finished'] = 1;
  }
}

function sample_migration_title_body_paragraphs_finished() {
  $messenger = \Drupal::messenger();
  if ($success) {
    $message = t('@count Reviews were added to Groups', ['@count' => count($results)]);
    $messenger->addMessage($message);
  }
  else {
    // Something went wrong
    $error_operation = reset($operations);
    $messenger->addMessage(
      t('An error occurred while processing @operation with arguments : @args',
        [
          '@operation' => $error_operation[0],
          '@args' => print_r($error_operation[0], TRUE)
        ]
      )
    );
  }
}

function sample_migration_get_title_body_field_collections() {
  Database::setActiveConnection('drupal7');
  $drupal7db = Database::getConnection();
  $query = $drupal7db->select('field_collection_item', 'fci')
    ->fields('fci', [
      'item_id',
      'field_name',
      'revision_id'
    ]);
  $query->innerJoin('field_data_' . $field_name, 'fd', 'fd.' . $field_name . '_value = fci.item_id');
  $query->fields('fd', [
    'entity_type',
    'bundle',
    'entity_id',
    $field_name . '_revision_id'
  ]);
  $query->condition('fci.field_name', $field_name);

  $query->leftJoin('field_data_field_usingpage_block_image', 'fdfubi', 'fdfubi.entity_id = fci.item_id');
  $query->fields('fdfubi', [
    'entity_id',
    'revision_id',
    'field_usingpage_block_image_fid'
  ]);

  $results = $query->execute()->fetchAll();
  Database::setActiveConnection('default');
  if (count($results)) {
    $drupal8db = Database::getConnection();
    $query = $drupal8db->select('migrate_map_upgrade_d7_node_using_library_page', 'mm');
    $query->fields('mm', [
      'sourceid1',
      'destid1'
    ]);
    $map_results = $query->execute()->fetchAllAssoc('sourceid1');
  }
}

function sample_migration_get_test_paragraph_content_map() {
  $drupal8db = Database::getConnection();
  $query = $drupal8db->select('migrate_map_upgrade_d7_node_using_library_page', 'mm');
  $query->fields('mm', [
    'sourceid1',
    'destid1'
  ]);
  $map_results = $query->execute()->fetchAllAssoc('sourceid1');
}