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Chapter 20. Toolset Guide

20.1. Automatic schema generation
20.1.1. Customizing the schema
20.1.2. Running the tool
20.1.3. Properties
20.1.4. Using Ant
20.1.5. Incremental schema updates
20.1.6. Using Ant for incremental schema updates
20.1.7. Schema validation
20.1.8. Using Ant for schema validation

Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and Ant tasks.

Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering of existing databases:

Please refer to the Hibernate Tools package documentation for more information.

However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka hbm2ddl.It can even be used from "inside" Hibernate.

DDL can be generated from your mapping files by a Hibernate utility. The generated schema includes referential integrity constraints, primary and foreign keys, for entity and collection tables. Tables and sequences are also created for mapped identifier generators.

You must specify a SQL Dialect via the hibernate.dialect property when using this tool, as DDL is highly vendor-specific.

First, you must customize your mapping files to improve the generated schema. The next section covers schema customization.

Many Hibernate mapping elements define optional attributes named length, precision and scale. You can set the length, precision and scale of a column with this attribute.

<property name="zip" length="5"/>
<property name="balance" precision="12" scale="2"/>

Some tags also accept a not-null attribute for generating a NOT NULL constraint on table columns, and a unique attribute for generating UNIQUE constraint on table columns.

<many-to-one name="bar" column="barId" not-null="true"/>
<element column="serialNumber" type="long" not-null="true" unique="true"/>

A unique-key attribute can be used to group columns in a single, unique key constraint. Currently, the specified value of the unique-key attribute is not used to name the constraint in the generated DDL. It is only used to group the columns in the mapping file.

<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
<property name="employeeId" unique-key="OrgEmployee"/>

An index attribute specifies the name of an index that will be created using the mapped column or columns. Multiple columns can be grouped into the same index by simply specifying the same index name.

<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>

A foreign-key attribute can be used to override the name of any generated foreign key constraint.

<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>

Many mapping elements also accept a child <column> element. This is particularly useful for mapping multi-column types:

<property name="name" type="my.customtypes.Name"/>
    <column name="last" not-null="true" index="bar_idx" length="30"/>
    <column name="first" not-null="true" index="bar_idx" length="20"/>
    <column name="initial"/>

The default attribute allows you to specify a default value for a column.You should assign the same value to the mapped property before saving a new instance of the mapped class.

<property name="credits" type="integer" insert="false">
    <column name="credits" default="10"/>
<version name="version" type="integer" insert="false">
    <column name="version" default="0"/>

The sql-type attribute allows the user to override the default mapping of a Hibernate type to SQL datatype.

<property name="balance" type="float">
    <column name="balance" sql-type="decimal(13,3)"/>

The check attribute allows you to specify a check constraint.

<property name="foo" type="integer">
    <column name="foo" check="foo > 10"/>
<class name="Foo" table="foos" check="bar < 100.0">
    <property name="bar" type="float"/>

The following table summarizes these optional attributes.

The <comment> element allows you to specify comments for the generated schema.

<class name="Customer" table="CurCust">
    <comment>Current customers only</comment>
<property name="balance">
    <column name="bal">
        <comment>Balance in USD</comment>

This results in a comment on table or comment on column statement in the generated DDL where supported.