I'm currently working on a Grails project which has a static production database with a lot of data in it. I would like to test my application using the production data, but instead of having to clone the production database I'd like to setup a proxy database to the production database.
Essentially reads of the database would go all the way to production database while writes would stop at a proxy database (preferably an h2 database). If a row was updated that came from the production database the row would be saved to the proxy database and returned, instead of the production's row, on subsequent queries.
I'd like to do all of this as transparently to the application as possible. My currently line of thinking is that I'd need to fork the Hibernate GORM implementation and make it support this use case. Has this been done before? Is there a better way?
Forking the Hibernate GORM implementation may not be a good idea. You will be stuck in your version and will have to, somehow, make this up to date with the original plugin (eg. bug fix, new implementations).
Maybe a custom TestMixin that allows you to override all registered domain classes, with new implementations of save(), get(), find() and etc can be an option. You can work with the metaClass to override this static methods and this will be triggered only on tests with the annotated mixin.
With this you can use multiple datasources in the test environment to determine which will be used.