views:

80

answers:

3

I've been briefly looking at JPA recently, and I was wondering what the deal is with database schema migrations and staying lined up with the classes you've created.

Is there support in JPA for this stuff? Utilities? Best Practises?

Cheers!

A: 

If you set the generateDdl to Hibernate (if it is the underlying implementation), then it generates the database schema according to your current dialect. So after changing the dialect it will automatically generate the database.

Other JPA providers may have different properties for this.

Bozho
This is correct, but it doesn't address the issue of schema migration.
HDave
@HDave - on the contrary. The schema is replicated to the new database, reflecting the current data model.
Bozho
By schema migration, I believe the OP means having scripts that can take an existing database with data and bring it up to date with the new entities/schema. Often this involves careful study of the schema changes and sometime the unload/reloading of data so that FK's can be changed, PK's can be split, etc. Hibernate can generate a script to make a new schema. Hibernate can also generate a script to modify an old schema to make it a new schema, but it assumes the database has no data -- and even then my understanding is that capability is seldom used because of "issues."
HDave
+1  A: 

The short answer is no.

If you change your beans, then you will have to migrate the existing schema by hand. So for Rails style database migrations you will have to look elsewhere.

You can however generate the initial ddl from your Java beans easily. The example below illustrates schema creation with EclipseLink version 2.0:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"&gt;
    <persistence-unit name="JPATestPU" transaction-type="RESOURCE_LOCAL">
        <provider>
            org.eclipse.persistence.jpa.PersistenceProvider
        </provider>
        <class>org.randompage.MyEntity</class>
        <properties>
            <property name="javax.persistence.jdbc.user" value="johndoe"/>
            <property name="javax.persistence.jdbc.password" value="secret"/>
            <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:h2:~/.h2/testdb;FILE_LOCK=NO"/>
            <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
            <property name="eclipselink.logging.level" value="INFO"/>
        </properties>
    </persistence-unit>
</persistence>

The key element here is

 <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/> 

This tells EclipseLink to drop existing tables and generate new once from your JPA mapping. This procedure is highly vendor specific so for other JPA vendors (Hibernate, OpenJPA...) you will have to consult their specific documentation.

Lars Tackmann
+2  A: 

I won't rely on JPA providers to update the database schema. Check Liquibase for one of the good approaches.

lexicore