Lack of primary key causes migrations to fail

Good point on removing the tid index. See updated patch.

Yes, this is a D6 to D7 upgrade. I agree, I would have thought it would be simple.

I have a straightforward mapping using MigrateDestinationTable, and a MigrateSQLMap using tid and rid as the keys:

    $query = $this->getConnection()
      ->select('forum_access', 'fa')
      ->fields('fa');

    $table_name = 'forum_access';

    $this->map = new MigrateSQLMap(
      $this->machineName,
      array(
        'tid' => array(
          'type' => 'int',
          'length' => 11,
          'unsigned' => TRUE,
          'not null' => TRUE,
          'default' => '0',
          'description' => 'Term ID.',
          'alias' => 'fa',
        ),
        'rid' => array(
          'type' => 'int',
          'length' => 11,
          'unsigned' => TRUE,
          'not null' => TRUE,
          'default' => '0',
          'description' => 'Role ID.',
          'alias' => 'fa',
        ),
      ),
      MigrateDestinationTable::getKeySchema($table_name)
    );

    $this->destination = new MigrateDestinationTable($table_name);

Have created migrations for many other tables, and forum_access is the first time I've run into any problem like this.

I also ran into a second migration issue regarding Forum Access that I'm going to post separately.

Cross-referencing the other forum_access issue I had when migrating: https://www.drupal.org/project/forum_access/issues/3003279

Good question, with a very straightforward reason.

These are not upgrades. We're using Drupal 6 sites as data repositories to import into a vastly different D7 solution. They will all be part of a single, consistent codebase.

Some of the sites were built by us, and some were built by others. Very little in the D6 sites match what we have in the new platform, so we are mapping what they do have into the proper fields, content types, etc.