Per-document migration

[nyarly]

I'm not exactly sure how this would work, but a significant feature for any ODM would be the ability to build migrations on a per-document scale.

I think this would amount to some means (akin to _type) of determining the migration level of a document, as well as a DSL for describing how to load old versions of the document. On save, migration to the current version would be complete.

Of concern is multiple persistence clients at different versions, as well as down-migration (I don't recognize this version number...)

[myronmarston]

FWIW, we have a mixin we've been using for this:

https://gist.github.com/2002463

I mentioned this to @seancribbs at one point (thing it might make sense to roll it into ripple) and he was open to the idea. I'm not doing as much with riak/ripple these days, though, so I doubt I'll have the time to do it.

[duff]

For the times when we've needed to do migrations, we've used zd, a gem @ntalbott made. Thought I'd mention it in case you find it useful:

https://github.com/ntalbott/zd

[ntalbott]

Yah, zd is total experiment-ware; I'm favoring an approach more like what @nyarly is proposing going forward. Along those lines, Braintree's strategy is pretty interesting: https://www.braintreepayments.com/braintrust/data-migrations-for-nosql-with-curator