Contact.php

<?php

namespace Drupal\my_custom_migration\Plugin\migrate\source;

use Drupal\migrate\Plugin\migrate\source\SqlBase;
use Drupal\migrate\row;

/**
 * Source plugin for company.
 *
 * @MigrateSource(
 *   id = "contact"
 * )
 */
class Contact extends SqlBase {

  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->select('contact', 'ct')
      ->fields('ct', array(
        'id',
        'name',
        'mobile_phone',
        'company'
      ));
    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function fields() {
    // TODO: Implement fields() method.
    $fields = array(
      'id' => $this->t('ID'),
      'name' => $this->t('Contact Name'),
      'mobile_phone' => $this->t('Mobile number'),
      'company' => $this->t('Company')
    );
    return $fields;
  }

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    //drush_print_r($row);

    return parent::prepareRow($row); // TODO: Change the autogenerated stub
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    // TODO: Implement getIds() method.
    return [
      'id' => [
        'type' => 'integer',
        'alias' => 'ct'
      ]
    ];
  }
}