How can I ensure that services occur in the same route using constraints?

Question

I'm trying to use Jsprit constraints to ensure that a solution route has a specific set of services.. Given the services [S1, S2, S3, ..., S10], I want to ensure that services [S2, S4, S6] occur in the same route..

For this, I am using HardRouteConstraint..

@Override
public boolean fulfilled(JobInsertionContext context) {

    // jobIds = [S2, S4, S6]

    Job thisJob = context.getJob();
    if (jobIds.contains(thisJob.getId())) {
        for (String jobId : jobIds) {
            VehicleRoute route = stateManager.getProblemState(stateManager.createStateId(jobId), VehicleRoute.class);
            // if the jobs are assigned to a different route, reject this route
            if (route != null && route != context.getRoute()) return false;
        }
    }
    return true;
}

This works partially fine.. If S2, S4 and S6 are part of the solution, they appear in a single route, and are not split between different routes..

The problem is that if I have a limited vehicle capacity (say, 3), Jsprit can possibly return a solution like:

Routes: [
    [S1, S2, S3]
    [S5, S7, S8]
    [S9, S10]
]
Unassigned Jobs: [S4, S6]

This is understandable, but not what I want.. I want that if a route includes S2, it should also include S4 and S6, in any order..

How can I ensure that a valid solution does not contain such routes: [X, S2, Y] or [S2, X, S4]..

Thanks, Asim