DB::QueryLog() only work after you execute the query $builder->get(). if you want to get the query before execute the query you can use $builder->toSql() method. this is the example how to get the sql and bind it:
$query = str_replace(array('?'), array('\'%s\''), $builder->toSql());
$query = vsprintf($query, $builder->getBindings());
$result = $builder->get();
or just make your query error like calling for unexist table or column, you will see the generated query in exception XD
Hope this works!