The paradigm (Web design portfolio) mismatch the data, to an application-dependent
The paradigm mismatch 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: public class User { private Set billingDetails; … } public class BillingDetails { private Set users; … } 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): 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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