qr code scaner java app Understanding object/relational persistence in Java

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Understanding object/relational persistence
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(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
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Problems relating to associations
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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:
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public class User { private Set billingDetails; ... } public class BillingDetails { private User user; ... }
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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
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The paradigm mismatch
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the data, to an application-dependent navigational model, a constrained view of the associations needed by this particular application. It isn t possible to determine the multiplicity of a unidirectional association by looking only at the Java classes. Java associations can have many-to-many multiplicity. For example, the classes could look like this:
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public class User { private Set billingDetails; ... } public class BillingDetails { private Set users; ... }
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Table associations, on the other hand, are always one-to-many or one-to-one. You can see the multiplicity immediately by looking at the foreign key definition. The following is a foreign key declaration on the BILLING_DETAILS table for a one-tomany association (or, if read in the other direction, a many-to-one association):
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USER_ID bigint foreign key references USERS
These are one-to-one associations:
USER_ID bigint unique foreign key references USERS BILLING_DETAILS_ID bigint primary key foreign key references USERS
If you wish to represent a many-to-many association in a relational database, you must introduce a new table, called a link table. This table doesn t appear anywhere in the domain model. For our example, if we consider the relationship between the user and the billing information to be many-to-many, the link table is defined as follows:
create table USER_BILLING_DETAILS ( USER_ID bigint foreign key references USERS, BILLING_DETAILS_ID bigint foreign key references BILLING_DETAILS, PRIMARY KEY (USER_ID, BILLING_DETAILS_ID) )
We discuss association and collection mappings in great detail in chapters 6 and 7. So far, the issues we ve considered are mainly structural. We can see them by considering a purely static view of the system. Perhaps the most difficult problem in object persistence is a dynamic problem. It concerns associations, and we ve already hinted at it when we drew a distinction between object network navigation and table joins in section 1.1.4, Persistence in object-oriented applications. Let s explore this significant mismatch problem in more depth.
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