HIbernate Basic O/R Mapping

Basic O/R Mapping

The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations, not table declarations.Object/relational mappings are usually defined in an XML document.

Lets kick off with an example mapping:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
      "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

        <class name="Cat" 
            table="cats"
            discriminator-value="C">
                
                <id name="id">
                        <generator class="native"/>
                </id>

                <discriminator column="subclass" 
                     type="character"/>

                <property name="weight"/>

                <property name="birthdate"
                    type="date" 
                    not-null="true" 
                    update="false"/>

                <property name="color"
                    type="eg.types.ColorUserType"
                    not-null="true"
                    update="false"/>

                <property name="sex"
                    not-null="true" 
                    update="false"/>

                <property name="litterId"
                    column="litterId"
                    update="false"/>

                <many-to-one name="mother"
                    column="mother_id"
                    update="false"/>

                <set name="kittens"
                    inverse="true"
                    order-by="litter_id">
                        <key column="mother_id"/>
                        <one-to-many class="Cat"/>
                </set>

                <subclass name="DomesticCat"
                    discriminator-value="D">

                        <property name="name" 
                            type="string"/>

                </subclass>

        </class>

        <class name="Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

(1) name (optional): The fully qualified Java class name of the persistent class (or interface). If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.

(2) table (optional - defaults to the unqualified class name): The name of its database table.

(3) discriminator-value (optional - defaults to the class name): A value that distiguishes individual subclasses, used for polymorphic behaviour. Acceptable values include null and not null.

(4) mutable (optional, defaults to true): Specifies that instances of the class are (not) mutable.

(5) schema (optional): Override the schema name specified by the root element.

(6) catalog (optional): Override the catalog name specified by the root element.

(7) proxy (optional): Specifies an interface to use for lazy initializing proxies. You may specify the name of the class itself.

(8) dynamic-update (optional, defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed.

(9) dynamic-insert (optional, defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

(10) select-before-update (optional, defaults to false): Specifies that Hibernate should never perform an SQL UPDATE unless it is certain that an object is actually modified. In certain cases (actually, only when a transient object has been associated with a new session using update()), this means that Hibernate will perform an extra SQL SELECT to determine if an UPDATE is actually required.

(11) polymorphism (optional, defaults to implicit): Determines whether implicit or explicit query polymorphism is used.

(12) where (optional) specify an arbitrary SQL WHERE condition to be used when retrieving objects of this class

(13) persister (optional): Specifies a custom ClassPersister.

(14) batch-size (optional, defaults to 1) specify a "batch size" for fetching instances of this class by identifier.

(15) optimistic-lock (optional, defaults to version): Determines the optimistic locking strategy.

(16) lazy (optional): Lazy fetching may be completely disabled by setting lazy="false".

(17) entity-name (optional, defaults to the class name): Hibernate3 allows a class to be mapped multiple times (to different tables, potentially), and allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamic models” and Chapter 18, XML Mapping for more information.

(18) check (optional): A SQL expression used to generate a multi-row check constraint for automatic schema generation.

(19) rowid (optional): Hibernate can use so called ROWIDs on databases which support. E.g. on Oracle, Hibernate can use the rowid extra column for fast updates if you set this option to rowid. A ROWID is an implementation detail and represents the physical location of a stored tuple.

(20) subselect (optional): Maps an immutable and read-only entity to a database subselect. Useful if you want to have a view instead of a base table, but don't. See below for more information.

(21) abstract (optional): Used to mark abstract superclasses in hierarchies