Question

I have a table Project with fields

ID
PROJECT_BASELINE_ATTRIBUTES_ID (FK for table PROJECT_BASELINE_ATTR)

This table has the following mapping

<hibernate-mapping package="com.initech.db.model">
    <class name="com.initech.db.model.Project" table="PROJECT">

        <id name="id" column="id" type="java.lang.Long">
            <meta attribute="use-in-tostring">true</meta>
            <generator class="sequence">
                <param name="sequence">PROJECT_ID_SEQ</param>
            </generator>
        </id>

        <many-to-one name="projectBaselineAttributes" column="PROJECT_BASELINE_ATTRIBUTES_ID" class="com.initech.db.model.ProjectBaselineAttributes" cascade="all" not-null="true">
            <meta attribute="use-in-tostring">true</meta>
        </many-to-one>^

     </class>
</hibernate-mapping>

The respective Project.java class has the fields

private Long id;
private ProjectBaselineAttributes projectBaselineAttributes;

Furthermore, I have a table PROJECT_BASELINE_ATTR, containing the field

ID

The Hibernate mapping:

<hibernate-mapping package="com.initech.db.model">
    <class name="com.initech.db.model.ProjectBaselineAttributes" table="PROJECT_BASELINE_ATTR">

        <id name="id" column="id" type="java.lang.Long">
            <generator class="sequence">
                <param name="sequence">PRO_BASE_ATTR_ID_SEQ</param>
            </generator>
        </id>

      <set name="projects" table="PROJECT" inverse="true" lazy="true" fetch="select">
          <key>
              <column name="PROJECT_BASELINE_ATTRIBUTES_ID" precision="22" scale="0" not-null="true" />
          </key>
          <one-to-many class="com.initech.db.model.Project" />
      </set>


    </class>
</hibernate-mapping>

The respective ProjectBaselineAttributes.java class has the fields

private Long id;
private Set projects = new HashSet();

The current mapping is not ideal, as the relationship between the two tables is actually one-to-one, but in the ProjectBaselineAttributes I have a set of Projects, even though there is always one Projet for one ProjectBaseLineattributes. What should the mapping look like for ProjectBaselineAttributes so that I can get the associated Project “singularly”, i.e. so that the class ProjectBaselineAttributes.java would look like this:

private Long id;
private Project project;

Answer 1

Here’s the section of the Hibernate documentation which details how to map such a bidirectional one-to-one association using a foreign key.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <many-to-one name="address" 
        column="addressId" 
        unique="true"
        not-null="true"/>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
   <one-to-one name="person" 
        property-ref="address"/>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )