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

<?php

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

use Drupal\Core\Routing\RouteMatchInterface;
use \Drupal\Core\Database\Database;

/**
 * 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:
  }
}

/**
 * Batch Process to migrate all Title Body Field Collections from Drupal 7
 * into the Title Body Paragraphs in Drupal 8
 * You can read more about Batches in Drupal 8 here:
 * https://api.drupal.org/api/drupal/core%21includes%21form.inc/group/batch/8.5.x
 *
 * @param [type] $context
 * @return void
 */
function sample_migration_title_body_paragraphs(&$context) {
  $entity_type_manager = \Drupal::service('entity_type.manager');
  /* @var NodeStorage $node_storage */
  $node_storage = $entity_type_manager->getStorage('node');

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

  if (!isset($context['sandbox']['progress'])) {
    $context['sandbox']['progress'] = 0;
    $context['sandbox']['current_index'] = 0;

    // Load all Current Nodes type of Page
    $all_pages = $node_storage->loadByProperties([
      'type' => 'page'
    ]);

    // Need to set a bunch of data on the sandbox key to keep it on every loop through the batch process
    $context['sandbox']['page_content'] = array_values($all_pages);
    $context['sandbox']['title_page_fc'] = sample_migration_get_title_body_field_collections();
    $context['sandbox']['test_paragraph_map'] = sample_migration_get_test_paragraph_content_map();

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

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

  // here is each loop through the batch process
  foreach ($indexes as $index) {
    // based on index get the current field collection item I'm on
    // use a bunch of ifs to make sure we can proceed without breaking everything
    $current_result = $context['sandbox']['title_page_fc'][$index];
    if (!is_null($current_result)) {
      $d7_nid = $current_result->entity_id;
      if (!is_null($context['sandbox']['title_page_fc'][$index])) {
        $d8_nid = $context['sandbox']['test_paragraph_map'][$d7_nid]->destid1;
        if (!is_null($d8_nid)) {
          // if we made it this far it's safe to try and create a paragraph
          // Once the paragraph data is setup pass it to our create paragraph function
          $paragraph_data = [
            'field_title' => $current_result->field_fc_title_value,
            'field_body' => [
              'value' => $current_result->field_fc_body_value,
              'format' => 'full_html',
            ],
            'field_drupal7_item_id' => $current_result->field_test_paragraphs_content_value,
            'parent_type' => 'node',
            'type' => 'title_body',
            'status' => '1',
            'parent_field_name' => 'field_content',
            'parent_id' => $d8_nid
          ];
          sample_migration_create_title_body_paragraph($current_result, $paragraph_data);
        }
      }
    }

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

  // 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;
  }
}

/**
 * Simple batch process finished function
 *
 * @param [type] $success
 * @param [type] $results
 * @param [type] $operations
 * @return void
 */
function sample_migration_title_body_paragraphs_finished($success, $results, $operations) {
  $messenger = \Drupal::messenger();
  if ($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)
        ]
      )
    );
  }
}

/**
 * Undocumented function
 *
 * @return void
 */
function sample_migration_get_title_body_field_collections() {
  Database::setActiveConnection('drupal7');
  $drupal7db = Database::getConnection();
  $query = $drupal7db->select('field_data_field_test_paragraphs_content', 'fc');
  $query->fields('fc', [
    'entity_id',
    'field_test_paragraphs_content_value',
    'delta'
  ]);
  $query->innerJoin('field_collection_item', 'fci', 'fc.entity_id = fci.item_id');
  $query->fields('fci', [
    'item_id',
    'field_name',
    'revision_id'
  ]);
  $query->innerJoin('field_data_field_fc_title', 'title', 'title.entity_id = fci.item_id');
  $query->fields('title', [
    'entity_type',
    'bundle',
    'entity_id',
    'field_fc_title_value'
  ]);
  $query->innerJoin('field_data_field_fc_body', 'body', 'body.entity_id = fci.item_id');
  $query->fields('body', [
    'entity_type',
    'field_fc_body_value',
    'field_fc_body_format',
    'bundle',
    'entity_id'
  ]);

  $results = $query->execute()->fetchAll();
  Database::setActiveConnection('default');
  return $results;
}

/**
 * Undocumented function
 *
 * @return void
 */
function sample_migration_get_test_paragraph_content_map() {
  $drupal8db = Database::getConnection();
  $query = $drupal8db->select('migrate_map_d7_node__test_paragraphs', 'mm');
  $query->fields('mm', [
    'sourceid1',
    'destid1'
  ]);
  $map_results = $query->execute()->fetchAllAssoc('sourceid1');
  return $map_results;
}

/**
 * Create / Update a paragraph based on the data passed into it
 *
 * @param [type] $current_result
 * @param [type] $paragraph_data
 * @return void
 */
function sample_migration_create_title_body_paragraph($current_result, $paragraph_data) {
  $paragraph_storage = \Drupal::entityTypeManager()->getStorage('paragraph');
  $node_storage = \Drupal::entityTypeManager()->getStorage('node');

  // check if paragraph already exists, if so we need to update it
  // we set field_drupal7_item_id on it so we can make sure we aren't duplicating paragraphs
  // so we don't build a million paragraphs
  $existing_paragraph = $paragraph_storage->loadByProperties([
    'field_drupal7_item_id' => $current_result->field_test_paragraphs_content_value
  ]);
  if (!is_null($existing_paragraph) && count($existing_paragraph)) {
    $this_paragraph = reset($existing_paragraph);
  }
  else {
    $this_paragraph = $paragraph_storage->create($paragraph_data);
    $this_paragraph->save();
  }

  // attach paragraph to node
  $parent_node = $node_storage->load($current_result->entity_id);
  if (!is_null($parent_node)) {
    // parent field name
    $field_name = 'field_content';

    // find all paragraphs already on the node so we don't add this one twice
    // also so we don't delete a paragraph by adding a new one
    $node_paragraphs = $parent_node->get($field_name)->getValue();
    $should_add_paragraph = true;
    if (count($node_paragraphs)) {
      foreach ($node_paragraphs as $node_paragraph) {
        if ($node_paragraph['target_id'] === $this_paragraph->id()) {
          // if this paragraph is alrady attached, there is no reason to attach it again
          $should_add_paragraph = false;
          break;
        }
      }
    }
    if ($should_add_paragraph) {
      // add this paragraph to the paragraphs array on the node and save it
      $node_paragraphs[] = [
        'target_id' => $this_paragraph->id(),
        'target_revision_id' => $this_paragraph->getRevisionId()
      ];
      $parent_node->set($field_name, $node_paragraphs);
      $parent_node->save();
    }
  }
  else {
    \Drupal::messenger()->addMessage(t("Unable to attach paragraph to content. Paragraph ID: " . $this_paragraph->id()), 'error');
    \Drupal::logger('joco_migrations')->error("Unable to attach paragraph to content. Paragraph ID: " . $this_paragraph->id());
  }
}