How to: complex logic in migration script

[Jeremy Townson]

I have a migration task involving the generation of thumbnail images from existing images in a mongo collection.

This seems beyond the capabilities of mongo javascript so a changeset js file presumably will not work.

What approach would others suggest?

It would be easy to write a Java class to do the thumbnail generation. Is there a way of hooking a java call into a mongeez changeset?

[Jeremy Townson]

Here is an approach that combines the normal mongeez changesets with supplementary ones, written in java and works well.

From mongo 3 onwards, map reduce does not work from javascript migration files, so the java approach may be useful in a wider range of migration scenarios.

public class DatabaseMaintenance {

    private List<SupplementaryChangeSet> supplementaryChangeSets;

    @Autowired
    public DatabaseMaintenance(SupplementaryChangeSets supplementaryChangeSets) {
        this.supplementaryChangeSets = supplementaryChangeSets.get();
    }

    public void perform(DB db) {

        runMongeezChangesets(db);

        runSupplementaryChangesets(db);
    }

    // Run conventional js or xml migration scripts
    private void runMongeezChangesets(DB db) {
        Mongeez mongeez = new Mongeez();
        mongeez.setMongo(db.getMongo());
        mongeez.setDbName(db.getName());
        mongeez.setFile(new ClassPathResource("db-maintenance/mongeez.xml"));
        mongeez.process();
    }

    // Run additional changesets that do not fit into js or xml form
    private void runSupplementaryChangesets(DB db) {

        MongeezDao dao = new MongeezDao(db.getMongo(), db.getName());

        for (SupplementaryChangeSet changeSet : supplementaryChangeSets) {

            if ( ! dao.wasExecuted(changeSet) && changeSet.perform(db)) {
                dao.logChangeSet(changeSet);
            }
        }
    }
}

public abstract class SupplementaryChangeSet extends ChangeSet {
    public abstract boolean perform(DB db);
}

public class SupplementaryChangeSets {

    @Resource
    private ThumbnailGenerationChangeSet thumbnailGenerationChangeSet;

    public List<SupplementaryChangeSet> get() {
        return asList(thumbnailGenerationChangeSet);
    }
}

@Component
public class ThumbnailGenerationChangeSet extends SupplementaryChangeSet {

    @Override
    public boolean perform(DB db) {

        // ... do stuff in java ...

    }
}

[Michael Kasch]

if it is stll intersting to you you should also have a look at: 
https://github.com/mongobee/mongobee they seem to have this as their design concept