Move to simpler vdm system

[rgrp]

Option 1: 'Changeset' Model

See ticket:1135 for vdm ticket. This would involve a) moving to changeset in vdm b) doing the migration in ckan to support this.

Have developed a new "changeset" based model for revisioning in vdm.

Implementation

• The main challenge with this change is schema and data migration

Every revisioned object has a revision_id and revision attribute.

Approximate algorithm:

Revision -> Changeset

for revtype in [PackageRevision, ...]:
    for pkgrev in package_revision:
        changeset = lookupchangeset(package_revision)
        ChangeObject(cset, (table, id), dictize(pkgrev))

Question:

• does pkg include tags attributes or not? or we have to dictize, pkgrev, pkg2tagrev, and tag. Probably the latter.

Option 2: Simplify Revision Object Model

Just use a simpler vdm, see ticket:1136 (move to SessionExtension) and ticket:1137 (remove need for statefulness in vdm).

Discussion

Advantage of Option 1 versus 2:

• Easier support for pending state and similar behaviour
• No need to introduce new tables (and hence migrations) when making something revisioned (or not).

Disadvantages

• Migration is required
• More difficult to query revision history.
  • Could be addressed by having ChangeObject have separate cols for table name and id but would likely be more difficult.
• Performance (?)
  • Have one big ChangeObject table to query when looking at changed objects rather than many revision tables.
    • Not sure this is a biggie as even with Revision model biggest revision object tables are probably on the order of the ChangeObject table

Conclusion

Implement Option 2 and leave Option 1 for present.

Option 1 includes Option 2 so it seems that that is required in either case (so we may as well with Option 2).

Option 1 requires significant effort (esp migration) so leave for present and then review the situation at some later date.