Criteria crit = session.createCriteria(User.class);
crit.createCriteria("departments").add(Expression.eq("departmentName", departmentName));
List result = crit.list();

net.sf.hibernate.AssertionFailure  an assertion failure occured (this may indicate a bug in Hibernate, but is more likely due to unsafe use of the session)
net.sf.hibernate.AssertionFailure  TRAS0014I: The following exception was logged net.sf.hibernate.AssertionFailure: not an association
   at net.sf.hibernate.collection.AbstractCollectionPersister.getElementPersister(AbstractCollectionPersister.java:707)
   at net.sf.hibernate.type.PersistentCollectionType.getAssociatedClass(PersistentCollectionType.java:221)
   at net.sf.hibernate.impl.CriteriaImpl.getClassForPath(CriteriaImpl.java:282)

<hibernate-mapping>
  <class name="com.giux.persistence.User"
    table="app_users"
    dynamic-update="false"
    dynamic-insert="false">

      <id
        name="id"
        column="id"
        type="java.lang.Long"
        unsaved-value="null">
          <generator class="native"></generator>
      </id>

      Blah, Blah property definitions

      <bag name="departments" table="app_usersdepartments" lazy="false" inverse="false">
        <key>
          <column name="userId" not-null="true"/>
        </key>
        <composite-element class="com.giux.persistence.UserDepartment">
          <property name="userName" type="string"></property>
          <property name="departmentName" type="string"></property>
      
          <many-to-one name="department" class="com.giux.persistence.Department">
            <column name="departmentId" not-null="true"/>
          </many-to-one>
        </composite-element>
      </bag>

    </class>

</hibernate-mapping>