Migrate from a CSV to content entities with Paragraphs

id,album_title,track_title,track_url
1,Axis: Bold As Love,Exp,https://www.deezer.com/fr/track/4952828
2,Axis: Bold As Love,Up From The Skies,https://www.deezer.com/fr/track/4952829
3,Axis: Bold As Love,Spanish Castle Magic,https://www.deezer.com/fr/track/4952830
4,Axis: Bold As Love,Wait Until Tomorrow,https://www.deezer.com/fr/track/4952832
5,Axis: Bold As Love,Aint No Telling,https://www.deezer.com/fr/track/4952831
...

id: track_paragraphs
label: Track Paragraphs
migration_group: discography

source:
  plugin: csv
  path: modules/custom/migrate_discography/data/album_tracks.csv
  header_row_count: 1
  keys:
    - id

process:
  field_title: track_title
  field_link:
    plugin: urlencode
    source: track_url

destination:
  plugin: entity_reference_revisions:paragraph
  default_bundle: track

migration_dependencies:
  required: {}
  optional: {}

dependencies:
  enforced:
    module:
      - migrate_discography

id: album_media
label: Album Media
migration_group: discography

source:
  plugin: url
  data_fetcher_plugin: file
  # Make use of a custom parser here, to convert the CSV
  # into associative arrays.
  data_parser_plugin: album_parser
  track_changes: true
  urls: modules/custom/migrate_discography/data/album_tracks.csv
  item_selector: /albums
  fields:
    - name: album_title
      label: Album title
      selector: album_title
    - # This field does not exist as is in the CSV
      # and is provided by the data parser.
      name: tracks
      label: Tracks
      selector: tracks
  ids:
    album_title:
      type: string

process:
  # Media name.
  name: album_title
  # Paragraphs field.
  field_tracks:
    plugin: sub_process
    source: tracks
    process:
      temporary_ids:
        plugin: migration_lookup
        migration: track_paragraphs
        # The id is the one from the CSV,
        # used to get the right paragraph.
        source: id
      target_id:
        plugin: extract
        source: '@temporary_ids'
        index:
          - 0
      target_revision_id:
        plugin: extract
        source: '@temporary_ids'
        index:
          - 1

destination:
  plugin: entity:media
  default_bundle: album

migration_dependencies:
  required:
    - track_paragraphs
  optional: {}

dependencies:
  enforced:
    module:
      - migrate_discography

<?php

namespace Drupal\migrate_discography\Plugin\migrate_plus\data_parser;

use Drupal\migrate_plus\Plugin\migrate_plus\data_parser\Json;

/**
 * Builds relations between Albums and Tracks
 * and dedupes Album entities from a flat CSV.
 * Then delegates to the Json data parser for the selectors.
 *
 * @DataParser(
 *   id = "album_parser",
 *   title = @Translation("Album parser")
 * )
 */
class AlbumParser extends Json {

  /**
   * {@inheritdoc}
   */
  protected function getSourceData($url) {
    // Get the CSV.
    $response = $this->getDataFetcherPlugin()->getResponseContent($url);
    // Convert the flat CSV into associative arrays.
    // 0 = Id
    // 1 = Album title
    // 2 = Track title
    // 3 = Track url
    $source_data = [
      'albums' => [],
    ];
    $lines = explode("\n", $response);
    // Exclude the first (header) row. Could be moved in config.
    array_shift($lines);
    $albumDetails = [];
    foreach ($lines as $line) {
      $csvLine = str_getcsv($line);
      if (!empty($csvLine[1])) {
        if (!array_key_exists($csvLine[1], $albumDetails)) {
          $albumDetails[$csvLine[1]] = [
            'album_title' => $csvLine[1],
            'tracks' => [],
          ];
        }
        $albumDetails[$csvLine[1]]['tracks'][] = [
          'id' => $csvLine[0],
        ];
      }
    }
    // In two times, to avoid key indexed results by product id.
    foreach ($albumDetails as $albumDetail) {
      $source_data['albums'][] = $albumDetail;
    }

    // Section from parent class.

    // Backwards-compatibility for depth selection.
    if (is_int($this->itemSelector)) {
      return $this->selectByDepth($source_data);
    }

    // Otherwise, we're using xpath-like selectors.
    $selectors = explode('/', trim($this->itemSelector, '/'));
    foreach ($selectors as $selector) {
      if (!empty($selector)) {
        $source_data = $source_data[$selector];
      }
    }
    return $source_data;
  }

}