public <V, RES extends Collection<V>> ResultTransformer<RES> collection(Supplier<RES> resultFactory, Expression<V> expression) {
        final Expression<V> lookup = getLookup(expression);
        return new GroupByGenericCollection<K, V, RES>(resultFactory, key, expression) {
            @Override
            protected V transform(Group group) {
                return group.getOne(lookup);
            }
        };
    }
Get the results as a set

@param expression projection
@return new result transformer
querydsl-core/src/main/java/com/querydsl/core/group/GroupByBuilder.java