Updates fail when table.inc is set to $this->systemOfRecord = Migration::DESTINATION

Two problems prevented me from performing updates to existing records in a table:

1. dbtng does not automatically set 'all_fields' unless $select->fields() is executed on the select object.
2. Keys are never populated, so db_write_record() always does an INSERT

      $select = db_select($this->tableName);
      // Below is needed; otherwise 'SELECT FROM' is sent, which results in an SQL error
      $select->fields($this->tableName, array());
      foreach ($this->schema['primary key'] as $key) {
        $select->condition($key, $entity->{$key});
        // Below is needed; otherwise $primary_key is left empty, leading db_write_record to perform
        // an insert.
        $primary_key[] = $key;
      }

So, either I'm missing something, or this is a bug. I'm happy to roll a patch if this seems right.