vr.filter = filter; vr.constraints = constraints;
    Relation getGroupbyPropertyRelation() {        if (groupby == null) {            throw new RuntimeException("getGroupbyPropertyRelation called on non-group-by relation");        }        Relation vr = new Relation(this);        vr.order = order;        vr.prototype = groupbyPrototype;        vr.filter = filter;        vr.constraints = constraints;        vr.addConstraint(new Constraint(null, groupby, true));        return vr;    }