In the other database systems I have worked with, it is common to have some means of migrating the schema of a database and the data along with it. As far as I know, in Elephant there is no standard way to define database versions and upgrades/downgrades to accompany them. Elephant could do with such a system.
In my use case, the main problem comes when a new class or slot is introduced, a slot is renamed, or slot properties change. I will make such a change in a development environment and then run a function that performs any necessary migration. I must then remember to run the migration function on the production database. For example, when a new slot is introduced it is unbound in existing instances. I will often set the slot value to nil in the migration function. Other times I will need to query the web for the correct value for existing objects' new slots. Defining a schema evolution system seems tricky now because the database schema is derived somewhat implicitly from defclass forms. Compare this scenario to Ruby on Rails where a series of migrations moves the database from a blank slate to a the latest and greatest with a series of migration scripts. The actual 'model' for a persistent Rails class is derived from the database itself, and the rails equivalent of defclass does not even list slots. I am not suggesting that we do things the Rails way, but I must say that the migration scheme is rather simple and powerful. The desire is to define a migration framework that satisfies the following criteria: * the database should keep track of the version of its schema. This need not be linear (see the difference between Ruby on Rails migrations in 2.0 and 2.1) * a developer should be able to introduce a new version of the database and know exactly how the schema changes * version upgrades should call hooks that migrate the database in light of the schema changes Perhaps the most straightforward implementation would be to introduce a new means of defining persistent classes and their indices more explicitly, instead of letting defclass do all the work. Instead of adding ":index t" to a slot definition, you would instead define a migration and call (add-index-for-slot (find-class 'user) 'telephone-number). Is there a means of achieving this already? If not, it would be a great feature to add and I would appreciate any comments. Best, Red _______________________________________________ elephant-devel site list elephant-devel@common-lisp.net http://common-lisp.net/mailman/listinfo/elephant-devel
