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Some items in listing 8.1 deserve explanation in greater detail. At the beginning of the build file we take advantage of XML entity references to share build file fragments with other build files. Entity reference includes are covered in more detail in chapter 9. All temporary build directories are deleted, even if they default to being physically under one another. We cannot assume that this default configuration is always the case. A user could override test.reports.dir, for example, to generate test reports to a different directory tree, perhaps under an intranet site.

public class User { private Set billingDetails; ... } public class BillingDetails { private Set users; ... }

Copying of non-.java files from the source tree to the compiled class directory is a common practice. Often property files or other metadata files live alongside source code. In our case, we have test cases that need known test data files. We keep them tightly coupled with our JUnit test case source code. XDoclet is used to generate a to-do list from @todo Javadoc comments and to dynamically construct a descriptor file making our custom tasks easier to integrate into build files. We cover these techniques in chapter 11. For the same reason we delete all temporary directories explicitly in our clean target, we create them individually here.

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):

<property name="env.COMPUTERNAME" value="${env.HOSTNAME}"/> <!-- ========================================================== --> platform <!-- Load property files --> machine <!-Note: the ordering is VERY important. --> name <!-- ========================================================== --> trick <property name="user.properties.file" Allow user location="${user.home}/.build.properties"/> properties to

These are one-to-one associations:

USER_ID bigint unique foreign key references USERS BILLING_DETAILS_ID bigint primary key foreign key references USERS

Cross-

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:

<!-- Load the application specific settings --> <property file="build.properties"/> <!-- Load user specific settings --> <property file="${user.properties.file}"/>

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) )

<!-- ========================================================== --> <!-- Directory mappings --> <!-- ========================================================== --> <property name="root.dir" location="${basedir}"/> <property name="masterbuild.dir" location="${root.dir}/.."/> <property file="${masterbuild.dir}/build.properties"/> <property name="src.dir" location="${root.dir}/src"/> <property name="build.dir" location="build"/> <property name="build.classes.dir" location="${build.dir}/classes"/> <property name="dist.dir" location="dist"/> <property name="dist.bin.dir" location="${dist.dir}/bin"/> <property name="doc.dir" location="doc"/> <property name="javadoc.dir" location="${doc.dir}/javadoc"/> property name="lib.dir" location="${masterbuild.dir}/lib"/>

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.

Legacy schemas sometimes have foreign key constraints that don t follow the simple FK references PK rule. Sometimes a foreign key references a nonprimary key: a simple unique column, a natural nonprimary key. Let s assume that in CaveatEmptor, you need to handle a legacy natural key column called CUSTOMER_NR on the USERS table:

<!-- ========================================================== --> <!-- Compile settings --> <!-- ========================================================== --> <property name="build.debug" value="on"/> Default compile settings <property name="build.optimize" value="off"/> <!-- ========================================================== --> <!-- Test settings --> <!-- ========================================================== -->

<class name="User" table="USERS"> <id name="id" column="USER_ID">...</id> <property name="customerNr" column="CUSTOMER_NR" not-null="true" unique="true"/> </class>

<property name="test.dir" location="${build.dir}/test"/> <property name="test.classes.dir" location="${test.dir}/classes"/> <property name="test.data.dir" location="${test.dir}/data"/> <property name="test.reports.dir" location="${test.dir}/reports"/> <property name="test.src.dir" location="${root.dir}/test"/> <property name="test.last.failed.file" location="${build.dir}/.lasttestsfailed"/>

The only thing that is probably new to you in this mapping is the unique attribute. This is one of the SQL customization options in Hibernate; it s not used at runtime (Hibernate doesn t do any uniqueness validation) but to export the database schema with hbm2ddl. If you have an existing schema with a natural key, you assume that it s unique. For completeness, you can and should repeat such important constraints in your mapping metadata maybe you ll use it one day to export a fresh schema. Equivalent to the XML mapping, you can declare a column as unique in JPA annotations: