Migrate D7 Field Collections to D8 Paragraphs

FieldCollectionItem.php

<?php

namespace Drupal\saintlukes_migrate\Plugin\migrate\source\d7;

use Drupal\migrate\Row;
use Drupal\migrate_drupal\Plugin\migrate\source\d7\FieldableEntity;

/**
 * Drupal 7 field_collection_item source from database.
 *
 * @MigrateSource(
 *   id = "d7_field_collection_item",
 *   source_provider = "field_collection"
 * )
 */
class FieldCollectionItem extends FieldableEntity {

  /**
   * {@inheritdoc}
   */
  public function query() {
    // Select node in its last revision.
    $query = $this->select('field_collection_item', 'fci')
      ->fields('fci', [
        'item_id',
        'revision_id',
        'field_name',
        'archived',
      ]);

    if (isset($this->configuration['field_name'])) {
      $query->innerJoin('field_data_' . $this->configuration['field_name'], 'fd', 'fd.' . $this->configuration['field_name'] . '_value = fci.item_id');
      $query->fields('fd', [
        'entity_type',
        'bundle',
        'entity_id',
        $this->configuration['field_name'] . '_revision_id',
      ]);
      $query->condition('fci.field_name', $this->configuration['field_name']);
    }
    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    // If field specified, get field revision ID so there aren't issues mapping.
    if (isset($this->configuration['field_name'])) {
      $row->setSourceProperty('revision_id', $row->getSourceProperty($this->configuration['field_name'] . '_revision_id'));
    }

    // Get Field API field values.
    foreach (array_keys($this->getFields('field_collection_item', $row->getSourceProperty('field_name'))) as $field) {
      $item_id = $row->getSourceProperty('item_id');
      $revision_id = $row->getSourceProperty('revision_id');
      $row->setSourceProperty($field, $this->getFieldValues('field_collection_item', $field, $item_id, $revision_id));
    }
    return parent::prepareRow($row);
  }

  /**
   * {@inheritdoc}
   */
  public function fields() {
    $fields = [
      'item_id' => $this->t('Item ID'),
      'revision_id' => $this->t('Revision ID'),
      'field_name' => $this->t('Name of field'),
      'entity_type' => $this->t('Host entity type'),
      'entity_id' => $this->t('Host entity ID'),
    ];
    return $fields;
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    $ids['item_id']['type'] = 'integer';
    $ids['item_id']['alias'] = 'fci';
    return $ids;
  }

}

migrate_plus.migrate.d7_field_collection_field_name.yml

id: d7_field_collection_FIELD_NAME
label: D7 Field Collection - Field Name
source:
  plugin: d7_field_collection_item
  field_name: FIELD_COLLECTION_FIELD_NAME
process:
  # Map fields from field collection to paragraph
  field_example: field_example
  type:
    plugin: default_value
    default_value: PARAGRAPH_TYPE
  revision_id: revision_id
destination:
  plugin: entity:paragraph

migrate_plus.migrate.d7_node_NODE_TYPE.yml

id: d7_node_NODE_TYPE
label: D7 Node - Node Type
source:
  plugin: d7_node
  node_type: D7_NODE_TYPE
process:
  PARAGRAPH_FIELD:
    plugin: iterator
    source: FIELD_COLLECTION_FIELD_NAME
    process:
      target_id:
        plugin: migration
        migration: d7_field_collection_FIELD_NAME
        source: value
      target_revision_id: revision_id
destination:
  plugin: entity:node
  default_bundle: NODE_BUNDLE
migration_dependencies:
  required:
    - d7_field_collection_FIELD_NAME