ProjectDoc projectDoc = new ProjectDoc();
            ProjectDocId docId = new ProjectDocId();
            DocCollection docCollection = new DocCollection();
            docCollection.setId(docColId);
            docId.setDocCollection(docCollection);
            docId.setProject(new Project(projId));
            projectDoc.setId(docId);
            session.saveOrUpdate(projectDoc);

org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing:

<hibernate-mapping>
    <class name="ProjectDoc" table="PROJECT_DOC" >
        <composite-id name="id" class="ProjectDocId">
            <key-many-to-one name="docCollection" class="DocCollection">
                <column name="ID" />
            </key-many-to-one>
            <key-many-to-one name="project" class="Project">
                <column name="ID" />
            </key-many-to-one>
        </composite-id>
        <many-to-one name="project" class="Project" update="false" insert="false" fetch="select">
            <column name="ID" not-null="true" />
        </many-to-one>
        <many-to-one name="docCollection" class="DocCollection" update="false" insert="false" fetch="select">
            <column name="ID" not-null="true" />
        </many-to-one>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="DocCollection" table="DOC_COLLECTION" >
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <version name="version" column="VERSION" insert="false" ></version>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="256" not-null="true" />
        </property>
     </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="Project" table="PROJECT" >
    <cache usage="nonstrict-read-write"/>
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="identity" />
        </id>
          <version name="version" column="VERSION" insert="false" ></version>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="256" not-null="true" />
        </property>
      </class>
</hibernate-mapping>