The model classes encapsulate knowledge about the database that
may change. That knowledge may be table names, or the value of
constants (such as `Group::MEMBER_ID` in this case).
Models may even disappear after a while due to refactorings.
In all these cases, this migration would break. Thus, we use
Laravel's query builder features which makes building queries
almost as easy as using the model - but correct in all cases. :-)