I've got a question on quite common practical problem - how to best represent derived fields?

Suppose I have a domain object Book which has one-to-many association to Rating object which represents 5-star rating value for the Book given by clients (much like in Amazon). I'd like to show Book info to clients along with associated average rating value. And here I have few options which I'd like to discuss.

1. Getting average rating for a particular book from DAO and storing it in presentation-only bean.
Book class doesn't have dedicated field for average rating and there is a method getAverageRating(Book) in BookDAO class which returns average rating for Book as result of HQL execution.
I don't like this approach as I have to introduce additional bean class to hold book along with its average rating to be used in presentation logic. Besides I feel like I'm violating rule that all data should be where it's processed.

2. Getting average rating for a particular book from DAO inside Book domain object.
As in previous option I have dedicated method in DAO and read only property (getter method) in Book class which delegates work to DAO.
With this option I'm getting rid of additional presentation-only bean but introduce dependency in domain object on DAO layer which I can't consider to be a best option (think of testing problems).

3. Injecting average rating to Book while getting it from DAO.
Method findBook() in DAO could return Book instance filled with average rating using setter method on bean.
I think it's quite viable option but maybe you have some thoughts on why it could be not.

4. Using derived property configured in mapping.
I can use "formula" element in mapping configuration for Book class to represent average rating property.
This is an option I'd rather use but as in previous case there could be nuances I don't know about.

So, dear Hibernate gurus, could you help me resolve my doubts?

P.S. I'd very appreciate any links on common Hibernate design patterns as I'm quite new with it.

Thank you, Oleg.

I'd recommend a hybrid of your last two approaches. Add an averageRating attribute to the Book class, mark it as transient and use something like this to lookup a Book instance with JPQL (I'm sure HQL supports something similar):



With a constuctor on Book to copy the persistent properties out.

This doesn't pollute your code with a bunch of helper objects and methods that only really care about the average rating, only requiring Book to initialize itself from another Book instance.

--
Tim