I have a confusion with entity classes. as far as I know entity classes are directly mapped to database entities (tables). I want to know in which classes business functions(eg. in a Library System, reserveBook(),checkoutBook() so on...) should be implemented. Do I need to put business functions to a set of separate classes or can I keep them in relevant entity classes? Actually I want to know what the best & accepted (If any standard exists) way of doing it. I hope it'll be really helpful if I give you an example.

For the simplicity, let's say that we're designing a library system. I want to know in which class reserveBook() method should be implemented. In Book entity class or In Member(entity) class or In a separate class reserved for business functionalities such as Services.

Some people say that reserveBook() should be implemented in Book(entity) class because it's a function relevant to book entity. Some guys say that it should be implemented in Member(entity) class since it's a function of a library member. Others believe that method should be implemented in a separate class likes a Service in order to keep Book(entity) class as a pure entity class that is directly mapped to a database table.

I would be grateful If you can help me to solve my problem using this little example since it seems to be more effective.

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suresh