We want an API that allows users to filter collections to subsets on the SQL level during the runtime (not using DQL).

Example:

class Post
{
   private $comments;
   public function getRecentComments()
   {
       return $this->comments->select(Expression::lessThan("commentDate", new \DateTime("-7 days")));
   }
}

The language of this Expressions for selecting entities out of a collection has to be so simple that it works on both arrays and any persistence backed data-source.

The Collection#select() method would return a new collection that is not equal the invoked collection.