It seems that some migrations are broken. For a short test I tried to migrate from version 19 down to version 1. Unfortunately, this was not possible anymore due to some errors in the migration functions.

It turned out that we sometimes use the find-method of the Schema-Classes directly which caused the error. Instead we should work on the find-method of the Collection directly (e.g. MinutesSchema.getCollection().find()) which in turn returns documents instead of astronomy classes.