The question is how sharply you can divide your objects into Value Objects and Service Objects. In ActiveRecord is hard to test Hevery proposes a design where the record in Active Record is a Value Object (newable in his terminology is a synonym) with no link to the repository object which would do all the database manipulation. This would work for normal attributes - you'd assign the value there on object creation - but what about relations?
$user->booksis very convenient, especially in templates, and it can be pure - if you know that you need the books on the user you could put them there when creating the user object. But then you'd have two kinds of objects - those with books, needed in some places - and those without, when you want to spare the additional database calls. Maybe we need subclasses of
User? But then we'd need subclasses for all possible combinations of relations (users with books, users with friends, users with books and friends, ...), not to mention the mess with relations on the related objects.