实现将业务逻辑排序/过滤到DDD聚合中

Question

I am fairly new to DDD and for me I search in practical terms to a good function for an aggregate. I have a long list of users (User) and a create an aggregate (UserAggregate) for that.

Now in my view I might display users based on different criteria. Let's assume here I don't sort the list in my database query, it is OK to have sorting in an aggregate? I cannot come up with another part of the application where I can do this properly, but I might misunderstand the function of an aggregate.

$users = new UserAggregate(array(
 $user1, $user2, $user3, $user4
));

foreach ($users->sortBySurname(), $user) {
 //
}

foreach ($users->sortByLastLoggedIn(), $user) {
 //
}

The same holds for filtering by the way. When I read about DDD aggregates, I immediately thought of the Doctrine\Common\Collections\Collection (link). That sort-of looks like an aggregate to me. So DDD experts, please enlighten me :-)

Update

The only way I was thinking btw was making aggregates rather immutual and have helpers to create new aggregates. But this seems not the way to go:

$aggregate = new UserAggregate(array($user1, $user2));
$helper    = new FilterSomething;
$aggregate = $helper->filter($aggregate);

The second aggregate is a new instance, so the helper looks like this:

class FilterSomething
{
  public function filter(Aggregate $aggregate)
  {
    $items = $aggregate->getItems();
    $items = $this->doFilter($items);

    return new Aggregate($items);
  }
}

Answer 1

A root aggregate is not a repository. A root aggregate represents a single user and all of it's domain specific information.

That sorting and filtering should be applied in a repository.