CHAPTER 1 Understanding object/relational persistence (if at all) (Abyss web server)
CHAPTER 1 Understanding object/relational persistence (if at all) should they be represented in the domain model? We discuss this question in chapter 4, section 4.2, Mapping entities with identity, and we find a solution with ORM. In the context of persistence, identity is closely related to how the system handles caching and transactions. Different persistence solutions have chosen different strategies, and this has been an area of confusion. We cover all these interesting topics and show how they re related in chapters 10 and 13. So far, the skeleton e-commerce application we ve designed has identified the mismatch problems with mapping granularity, subtypes, and object identity. We re almost ready to move on to other parts of the application, but first we need to discuss the important concept of associations: how the relationships between our classes are mapped and handled. Is the foreign key in the database all you need? 1.2.4 Problems relating to associations In our domain model, associations represent the relationships between entities. The User, Address, and BillingDetails classes are all associated; but unlike Address, BillingDetails stands on its own. BillingDetails instances are stored in their own table. Association mapping and the management of entity associations are central concepts in any object persistence solution. Object-oriented languages represent associations using object references; but in the relational world, an association is represented as a foreign key column, with copies of key values (and a constraint to guarantee integrity). There are substantial differences between the two representations. Object references are inherently directional; the association is from one object to the other. They re pointers. If an association between objects should be navigable in both directions, you must define the association twice, once in each of the associated classes. You ve already seen this in the domain model classes: public class User { private Set billingDetails; … } public class BillingDetails { private User user; … } On the other hand, foreign key associations aren t by nature directional. Navigation has no meaning for a relational data model because you can create arbitrary data associations with table joins and projection. The challenge is to bridge a completely open data model, which is independent of the application that works with
You want to have a cheap webhost for your apache application, then check apache web hosting services.