■ Table per concrete class with implicit polymorphism—Use no explicit
inheritance mapping, and default runtime polymorphic behavior.
■ Table per concrete class—Discard polymorphism and inheritance relationships completely from the SQL schema.
<hibernate-mapping>
<class
name="BillingDetails"
abstract="true">
<id
name="id"
column="BILLING_DETAILS_ID"
type="long">
<generator class="native"/>
</id>
<property
name="name"
column="OWNER"
type="string"/>
...
<union-subclass
name="CreditCard" table="CREDIT_CARD">
<property name="number" column=”NUMBER”/>
<property name="expMonth" column="EXP_MONTH"/>
<property name="expYear" column="EXP_YEAR"/>
</union-subclass>
<union-subclass
name="BankAccount" table="BANK_ACCOUNT">
...
</class>
</hibernate-mapping>
■ Table per class hierarchy—Enable polymorphism by denormalizing the SQL
schema, and utilize a type discriminator column that holds type information.
<hibernate-mapping>
<class
name="BillingDetails"
table="BILLING_DETAILS">
<id
name="id"
column="BILLING_DETAILS_ID"
type="long">
<generator class="native"/>
</id>
<discriminator
column="BILLING_DETAILS_TYPE"
type="string"/>
<property
name="owner"
column="OWNER"
type="string"/>
...
<subclass
name="CreditCard"
discriminator-value="CC">
<property name="number" column="CC_NUMBER"/>
<property name="expMonth" column="CC_EXP_MONTH"/>
<property name="expYear" column="CC_EXP_YEAR"/>
</subclass>
</class>
</hibernate-mapping>
Columns for properties declared by subclasses
must be declared to be nullable.
■ Table per subclass—Represent is a (inheritance) relationships as has a (for-
eign key) relationships.
<hibernate-mapping>
<class
name="BillingDetails"
table="BILLING_DETAILS">
<id
name="id"
column="BILLING_DETAILS_ID"
type="long">
<generator class="native"/>
</id>
<property
name="owner"
column="OWNER"
type="string"/>
...
<joined-subclass
name="CreditCard"
table="CREDIT_CARD">
<key column="CREDIT_CARD_ID"/>
<property name="number" column="NUMBER"/>
<property name="expMonth" column="EXP_MONTH"/>
<property name="expYear" column="EXP_YEAR"/>
</joined-subclass>
<joined-subclass
name="BankAccount"
table="BANK_ACCOUNT">
...
</class>
</hibernate-mapping>
■ If you do require polymorphic associations (an association to a superclass,
hence to all classes in the hierarchy with dynamic resolution of the concrete
class at runtime) or queries, and subclasses declare relatively few properties
(particularly if the main difference between subclasses is in their behavior),
lean toward table-per-class-hierarchy. Your goal is to minimize the number
of nullable columns and to convince yourself (and your DBA) that a denor-
malized schema won’t create problems in the long run.
■ If you do require polymorphic associations or queries, and subclasses
declare many properties (subclasses differ mainly by the data they hold),
lean toward table-per-subclass.
By default, choose table-per-class-hierarchy only for simple problems. For more
complex cases (or when you’re overruled by a data modeler insisting on the
importance of nullability constraints and normalization), you should consider the
table-per-subclass strategy. But at that point, ask yourself whether it may not be
better to remodel inheritance as delegation in the object model. Complex inherit-
ance is often best avoided for all sorts of reasons unrelated to persistence or ORM.
concept about custom mapping types: UserType, CompositeUserType…
<list name="images" table="ITEM_IMAGE">
<key column="ITEM_ID"/>
<list-index column="POSITION"/>
<element type="string" column="FILENAME" not-null="true"/>
</list>
对于排序collection有两种思路：
1.
<map name="images"
table="ITEM_IMAGE"
sort="natural">
private SortedMap images = new TreeMap();
2.
<map name="images"
table="ITEM_IMAGE"
order-by="IMAGENAME asc">
Collections of components:
<set name="images"
table="ITEM_IMAGE"
order-by="IMAGENAME asc">
<key column="ITEM_ID"/>
<composite-element class="Image">
<property name="name" column="IMAGENAME" not-null="true"/>
<property name="filename" column="FILENAME" not-null="true"/>
<property name="sizeX" column="SIZEX" not-null="true"/>
<property name="sizeY" column="SIZEY" not-null="true"/>
</composite-element>
</set>
