Describe potential solution to https://www.drupal.org/project/migrate_plus/issues/2949271

C5table.php

<?php

namespace Drupal\capacitype_move2aa\Plugin\migrate\destination;

use Drupal\Core\Database\Connection;
use Drupal\Core\Database\Database;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateSkipProcessException;
use Drupal\migrate\Plugin\MigrationInterface;
use Drupal\migrate\Plugin\migrate\destination\DestinationBase;
use Drupal\migrate\Row;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides table destination plugin.
 *
 * Use this plugin for a table not registered with Drupal Schema API.
 *
 * @MigrateDestination(
 *   id = "c5table"
 * )
 */
class C5table extends DestinationBase implements ContainerFactoryPluginInterface {

  /**
   * The name of the destination table.
   *
   * @var string
   */
  protected $tableName;

  /**
   * IDMap compatible array of id fields.
   *
   * @var array
   */
  protected $idFields;

  /**
   * Array of fields present on the destination table.
   *
   * @var array
   */
  protected $fields;

  /**
   * Represents a map of destination keys to source id keys.
   * eg. destination key is id, source key is nid : $mapping_of_keys['id'] = 'nid';
   *
   * @var array
   */
  protected $idFieldsMap;

  /**
   * The database connection.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $dbConnection;

  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, Connection $connection) {
    parent::__construct($configuration, $plugin_id, $plugin_definition, $migration);
    $this->dbConnection = $connection;
    $this->tableName = $configuration['table_name'];
    $this->idFields = $configuration['id_fields'];
    $this->fields = isset($configuration['fields']) ? $configuration['fields'] : [];
    $this->idFieldsMap = isset($configuration['id_fields_map']) ? $configuration['id_fields_map'] : [];
    $this->supportsRollback = TRUE;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    $db_key = !empty($configuration['database_key']) ? $configuration['database_key'] : NULL;

    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $migration,
      Database::getConnection('default', $db_key)
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    if (empty($this->idFields)) {
      throw new MigrateException('Id fields are required for a table destination');
    }
    return $this->idFields;
  }

  /**
   * {@inheritdoc}
   */
  public function fields(MigrationInterface $migration = NULL) {
    return $this->fields;
  }

  /**
   * {@inheritdoc}
   *
   * NOTE: This assumes only one sourceid1 key and only one destid1
   * @TODO more rigorous algorithm.
   */
  public function import(Row $row, array $old_destination_id_values = []) {
    $source_id_values = $row->getSourceIdValues();
    $primary_source_id = array_keys($source_id_values)[0];
    $primary_source_id_value = array_values($source_id_values)[0];
    $destination_id_values = [];
    if (count($source_id_values) != count($this->idFields)) {
      throw new MigrateSkipProcessException('All the id fields are required for a table migration.');
    }
    $values = $row->getDestination();
    $migrate_map_table = 'migrate_map_' . $this->migration->id();
    if ($this->fields) {
      $values = array_intersect_key($values, $this->fields);
    }
    $is_new = (empty($old_destination_id_values)) ? TRUE: FALSE;
    if ($is_new == FALSE) {
      $destination_id_value = db_query("SELECT destid1 FROM {$migrate_map_table} WHERE sourceid1 = :sid1", [':sid1' => $primary_source_id_value])->fetchField();
    }
    if (empty($destination_id_value)) {
      $is_new = TRUE;
    }
    $feedback = NULL;
    if ($is_new) {
      $last_insert_id = $this->dbConnection->insert($this->tableName)->fields($values)->execute();
      $feedback[$primary_source_id] = $last_insert_id;
    }
    else {
      if ($this->idFieldsMap) {
        $primary_destination_id = $this->idFieldsMap[$primary_source_id];
        $destination_id_value = db_query("SELECT destid1 FROM {$migrate_map_table} WHERE sourceid1 = :sid1", [':sid1' => $primary_source_id_value])->fetchField();
        $destination_id_values[$primary_destination_id] = $destination_id_value;
        $feedback[$primary_source_id] = $destination_id_value;
        $status = $this->dbConnection->merge($this->tableName)
          ->key($destination_id_values)
          ->fields($values)
          ->execute();
      }
      else {
        throw new MigrateSkipProcessException('Unable to find map between keys of source ids and destination ids.');
      }
    }
    return $feedback;
  }

  /**
   * {@inheritdoc}
   */
  public function rollback(array $destination_identifier) {
    $delete = $this->dbConnection->delete($this->tableName);
    foreach ($destination_identifier as $field => $value) {
      $delete->condition($field, $value);
    }
    $delete->execute();
  }
}

migrate_plus.migration.group2model.yml

id: group2model
label: "Asset Nodes to asset model."
migration_group: d82tables
dependencies:
  enforced:
    module:
      - capacitype_move2aa
langcode: en
status: true
source:
  plugin: c5ontent_entity:taxonomy_term
  bundle: group
  include_translations: false
process:
  name: 'name/0/value'
  description: 'description/0/value'
  created_at:
    plugin: c5_timestamp_to_datetime
    source: 'changed/0/value'
  updated_at:
    plugin: c5_timestamp_to_datetime
    source: 'changed/0/value'
destination:
   plugin: c5table
   table_name: groups
   database_key: aa
   id_fields:
      id:
        type: integer
   id_fields_map:
      tid: id
   fields:
      id: 'id'
      name: 'name'
      description: 'description'
      alias: alias
      url: url
      created_at: created_at
      updated_at: updated_at