How to update a database schema without losing your data with Hibernate?
Imagine you are developing a Java EE app using Hibernate and JBoss. You have a running server that has some important data on it. You release the next version of the app once in a while (1-2 weeks) and they have a bunch of changes in the persistence layer:
- New entities
- Removed entities
- Attribute type changes
- Attribute name changes
- Relationship changes
How do you effectively set up a system that updates the database schema and preserves the data? As far as I know (I may be mistaking), Hibernate doesn't perform alter column, drop/alter constraint.
Thank you, Artem B.
Answer
LiquiBase is your best bet. It has a hibernate integration mode that uses Hibernate's hbm2ddl to compare your database and your hibernate mapping, but rather than updating the database automatically, it outputs a liquibase changelog file which can be inspected before actually running.
While more convenient, any tool that does a comparison of your database and your hibernate mappings is going to make mistakes. See http://www.liquibase.org/2007/06/the-problem-with-database-diffs.html for examples. With liquibase you build up a list of database changes as you develop in a format that can survive code with branches and merges.