This post describes a design for implementing backwards-compatibility in deltashell.

Backwards-compatibility : The ability to open project files written by an older version of the application in the current application.

Global idea / setup

The idea is to use custom hbm.xml mapping to convert older project databases to the current object model. For example a plugin has made a release 1.0 which should be supported. Now if the mappings change during further development something has to be done to read the old projects. We need specific mappings to read the old files next to the current mappings. The situation would be as follows:

So the Person.hbm.1.0.xml describes the mapping of files written with 1.0 to the current version of the person. This mapping wil change as the Person evolves and we still want to support read old project files. NHibernate has a rich feature-set to get the data from the old schema to the new objects such as formulas, custom sql, usertypes etc. Another advantage of this approach is that backwards compatibility is solved using stand NHibernate technology and not DS specific stuff.

What happens when an old project gets openened?

If Deltashell opens an old project it creates a session for that version of the DB. After that all objects are migrated to a new session with the current configuration. Then when the project is saved the database is in a new format.