how to do Magma Class Evolution?

[Bart Gauquie]

Dear all,

I'm trying out the Class Evolution features of Magma. Typically you want to
have following stuff working: rename class, rename field, totally change
internal structure of some part of the domain. You do this during
development and have to make sure the data in production database gets
converted to these new changes.
I started by testing the 'rename class scenario': You have a running
production system with a filled database containing objects of a certain
type. During development, a class is renamed. The first thing I tried was to
load the package containing the new class type (and not the old type
anymore) in the production image. As promised on
http://wiki.squeak.org/squeak/5602, under implementation, Magma recreates
the old class definition in the production image. However, the objects
defined in the database are not automatically converted to the new classes.
(the rename of the class happened in the developer image, not in the
production image); meaning that if you add new entries, objects of both
class types will be in the database (which is bad).

To convert the old classes in database to the new classes, I tried the
update strategies, as mentioned on http://wiki.squeak.org/squeak/5602.
Working out the second option: one-shot script to enumerate all instances
and populate, I followed these steps (my domain is about Productbacklogs,
sprints and UserStories):

   - Userstory >> various >> copy: UserStoryOld (make a copy of the old
   UserStory)
   - UserStory >> refactor class >> rename: UserStoryNew (give the UserStory
   the new name)
   - UserStoryOld >> refactor class >> rename: UserStory (now you have a
   copy of the class definition of production with the old name)
   - write a script which converts all entries in the database using
   UserStory to UserStoryNew

   UserStory>>asUserStoryNEW
      |result|
      result := UserStoryNew new
      businessvalue: self businessvalue;
      contents: self contents;
      title: self title;
      yourself.
      resterendeTijdenNodig do: [:each|
       result addResterendeTijdNodig: each].
     ^result

   and:

   a script which iterates over all userstories in the database, creates
   UserStoryNew's from them, makes sure all links from and to the old
   UserStories are replaced by the new ones (for instance: put them in a
   productbacklog, replan them, ...); saving each of them again.
   - Load this package in the production image, run the script => all old
   entries are replaced with new classes.

This approach works. It has the advantage that every change is committed,
you can rollback to a commit point if something fails, and so forth. But it
takes a lot of time to do such a simple change? Is this what is meant by a
'one-shot script to enumerate all instances and populate' ? Is there another
way of doing this change? Because it would mean that for every rename of a
class you would have to write such a script.
A field rename can be handled in the same way. Let the old and new field
coexist, a script iterates over all the objects, copying the old field to
the new one. If it is just a simple type (Integer, Boolean or String), this
will be simple again, however if this field is something complex with a lot
of pointers from and to it, it will become somewhat more difficult to
correctly write a convert script. But it will be possible.

According to the wiki there is also 'a faster one-shot script that operates
directly on the MaObjectBuffers using MagmaFileTraverser'. I looked into
this, but I did not understand it. Any pointers how to, for instance, do a
class rename using this technique?


Thanks for any advice,

Kind Regards,

Bart

-- 
imagination is more important than knowledge - Albert Einstein
Logic will get you from A to B. Imagination will take you everywhere -
Albert Einstein
Learn from yesterday, live for today, hope for tomorrow. The important thing
is not to stop questioning. - Albert Einstein
The true sign of intelligence is not knowledge but imagination. - Albert
Einstein
However beautiful the strategy, you should occasionally look at the results.
- Sir Winston Churchill
It's not enough that we do our best; sometimes we have to do what's
required. - Sir Winston Churchill
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