如果将歧视标签在休眠状态下的位置向下移动，则会发生异常

Question

I am new to Hibernate. I am trying to map both my super-class and sub-class to a single table.

<class name="Employee" table="EmpWithManager">
        <id name="id" column="ID">
            <generator class="native"></generator>
        </id>
        <discriminator column="EMP_TYPE" type="string"></discriminator>
        <property name="firstName" column="FIRST_NAME"></property>
        <property name="lastName" column="LAST_NAME"></property>
        <property name="salary" column="SALARY"></property>


        <subclass name="Manager" extends="Employee">
            <property name="managerId" column="MAN_ID"></property>
            <property name="noOfEmployees" column="NUMBER_EMP"></property>
        </subclass>

    </class>

This works fine but if change the position of the discriminator tag as follows:

<class name="Employee" table="EmpWithManager">
    <id name="id" column="ID">
        <generator class="native"></generator>
    </id>

    <property name="firstName" column="FIRST_NAME"></property>
    <discriminator column="EMP_TYPE" type="string"></discriminator>
    <property name="lastName" column="LAST_NAME"></property>
    <property name="salary" column="SALARY"></property>


    <subclass name="Manager" extends="Employee">
        <property name="managerId" column="MAN_ID"></property>
        <property name="noOfEmployees" column="NUMBER_EMP"></property>
    </subclass>

</class>

This re-ordering gives me the below exception:

Caused by: org.xml.sax.SAXParseException: The content of element type "class" must match "(meta*,subselect?,cache?,synchronize*,comment?,tuplizer*,(id|composite-id),discriminator?,natural-id?,(version|timestamp)?,(property|many-to-one|one-to-one|component|dynamic-component|properties|any|map|set|list|bag|idbag|array|primitive-array)*,((join*,subclass*)|joined-subclass*|union-subclass*),loader?,sql-insert?,sql-update?,sql-delete?,filter*,fetch-profile*,resultset*,(query|sql-query)*)".

Please anybody tell me why this is happening and whether the position of discriminator should be in the beginning?

Answer 1

According to the hibernate document type definitions (DTD) listed here , the position of the discriminator tag must be after the id tag. Essentially the structure of the xml document in this situation is pre-defined, and you must follow the pre-defined format, and that is why you see an error after moving the discriminator tag.

From the JBoss docs: 5.1.8 - Discriminator :

The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy and declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types may be used: string, character, integer, byte, short, boolean, yes_no, true_false.

I'd imagine that you must define how properties will be discriminated against before you define them and that is the reasoning for the structure within the DTD.

Answer 2

If you look at the http://hibernate.org/dtd/ entry for hibernate-mapping-3.0.dtd it defines the class element as follows. Order is important as this is a DTD. Note that discriminator? comes after (id|composite-id) and before the long entry with property . This ordering requirement is not explicitly mentioned in the (current) hibernate documentation.

<!ELEMENT class (
    meta*,
    subselect?,
    cache?,
    synchronize*,
    comment?,
    tuplizer*,
    (id|composite-id),
    discriminator?,
    natural-id?,
    (version|timestamp)?,
    (property|many-to-one|one-to-one|component|dynamic-component|properties|any|map|set|list|bag|idbag|array|primitive-array)*,
    ((join*,subclass*)|joined-subclass*|union-subclass*),
    loader?,sql-insert?,sql-update?,sql-delete?,
    filter*,
    fetch-profile*,
    resultset*,
    (query|sql-query)*
)>