[dev] Optional migrations

[Chuck Hagenbuch]

Quoting Michael Rubinsky <mrubinsk at horde.org>:

> Maybe we need a generic way to separate out data conversion  
> migrations vs simple schema migrations for instances when it's  
> possible to switch back and forth between drivers? i.e.,The only  
> time data migration should be included is when it's mandatory, like  
> if the structure of one of the drivers changes, not when adding an  
> additional driver. The automatic migrations would only add the new  
> tables, and possibly run the initial conversion, while leaving the  
> old data intact. An additional migration script could be provided to  
> go from one driver to another. That way data conversions could be  
> run separately, and wouldn't be counted towards the schema version.  
> Maybe a post-install action could ask if the driver should be  
> switched? Any additional *mandatory* data conversion would be put  
> into the main-line migrations, and updated data conversion  
> migrations could be provided.

That sounds about right to me... I don't think that switching between  
drivers should be handled by the automatic migration system.

-chuck