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| Module | Purpose |
|--------|---------|
| `migrate` | Core migration framework |
| `migrate_drupal` | D6/D7 migration support |
| `migrate_drupal_ui` | Browser-based migration wizard |
| `migrate_plus` | Config-based migrations, extra source plugins |
| `migrate_tools` | Drush commands for migrations |
| `migrate_file` | File migration handling |

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// settings.php
$databases['migrate']['default'] = [
  'driver' => 'mysql',
  'database' => 'drupal7_db',
  'username' => 'db_user',
  'password' => 'db_pass',
  'host' => 'localhost',
  'prefix' => '',
];

drush en migrate_drupal_ui

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id: import_api_users
label: 'Import users from API'

source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  urls:
    - 'https://api.example.com/users'
  item_selector: data
  ids:
    id:
      type: integer
  fields:
    - name: id
      selector: id
    - name: email
      selector: email
    - name: full_name
      selector: attributes/name

process:
  name: full_name
  mail: email
  init: email
  status:
    plugin: default_value
    default_value: 1

destination:
  plugin: 'entity:user'

process:
  # Direct mapping
  title: source_title

  # Default value
  status:
    plugin: default_value
    default_value: 1

  # Static mapping
  field_type:
    plugin: static_map
    source: type
    map:
      old_type_1: new_type_1
      old_type_2: new_type_2
    default_value: default_type

  # Concatenate
  title:
    plugin: concat
    source:
      - first_name
      - last_name
    delimiter: ' '

  # Substring
  field_summary:
    plugin: substr
    source: body
    start: 0
    length: 200

process:
  # Migration lookup (referenced entity was migrated)
  uid:
    plugin: migration_lookup
    migration: users
    source: author_id

  # Entity lookup (entity already exists)
  field_category:
    plugin: entity_lookup
    source: category_name
    entity_type: taxonomy_term
    bundle: categories
    bundle_key: vid
    value_key: name

  # Entity generate (create if not exists)
  field_tags:
    plugin: entity_generate
    source: tags
    entity_type: taxonomy_term
    bundle: tags
    bundle_key: vid
    value_key: name

process:
  # Handle multiple values
  field_tags:
    plugin: sub_process
    source: tags
    process:
      target_id:
        plugin: entity_generate
        source: name
        entity_type: taxonomy_term
        bundle: tags
        value_key: name

  # Explode string to array
  field_keywords:
    - plugin: explode
      source: keywords
      delimiter: ','
    - plugin: entity_generate
      entity_type: taxonomy_term
      bundle: keywords
      value_key: name

process:
  # Skip if empty
  field_image:
    plugin: skip_on_empty
    method: process
    source: image_url

  # Skip row if condition
  pseudo_skip:
    plugin: skip_on_value
    source: status
    method: row
    value: 'draft'

<?php

declare(strict_types=1);

namespace Drupal\my_module\Plugin\migrate\source;

use Drupal\migrate\Attribute\MigrateSource;
use Drupal\migrate\Plugin\migrate\source\SqlBase;
use Drupal\migrate\Row;

/**
 * Custom source for legacy products.
 */
#[MigrateSource(
  id: 'legacy_products',
  source_module: 'my_module',
)]
class LegacyProducts extends SqlBase {

  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->select('legacy_products', 'p');
    $query->fields('p', ['id', 'name', 'price', 'description']);
    $query->condition('p.status', 'active');
    $query->orderBy('p.id');
    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function fields() {
    return [
      'id' => $this->t('Product ID'),
      'name' => $this->t('Product name'),
      'price' => $this->t('Price'),
      'description' => $this->t('Description'),
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    return [
      'id' => [
        'type' => 'integer',
        'alias' => 'p',
      ],
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    // Add computed fields or modify data
    $price = $row->getSourceProperty('price');
    $row->setSourceProperty('price_with_tax', $price * 1.21);

    return parent::prepareRow($row);
  }

}

<?php

declare(strict_types=1);

namespace Drupal\my_module\Plugin\migrate\process;

use Drupal\migrate\Attribute\MigrateProcess;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Converts price from cents to decimal.
 *
 * Example usage:
 * @code
 * process:
 *   field_price:
 *     plugin: cents_to_decimal
 *     source: price_cents
 * @endcode
 */
#[MigrateProcess(id: 'cents_to_decimal')]
class CentsToDecimal extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (empty($value) || !is_numeric($value)) {
      return NULL;
    }

    return number_format((float) $value / 100, 2, '.', '');
  }

}

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drush migrate:import migration_id -vvv

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// In custom process plugin
\Drupal::logger('my_migration')->notice('Processing: @value', ['@value' => $value]);

drush migrate:reset-status migration_id

drush migrate:import migration_id --limit=500

**Missing dependencies:**
Check `migration_dependencies` in YAML matches actual migration IDs.

my_migration/
├── my_migration.info.yml
├── my_migration.module
├── config/
│   └── install/
│       ├── migrate_plus.migration_group.my_group.yml
│       ├── migrate_plus.migration.users.yml
│       └── migrate_plus.migration.content.yml
├── src/
│   └── Plugin/
│       └── migrate/
│           ├── source/
│           │   └── LegacyProducts.php
│           └── process/
│               └── CentsToDecimal.php
└── data/
    └── import.csv