1.15.0

Improved refresh - it now works with prefetched objects. For example:

>>> band = await Band.objects(Band.manager).first()
>>> band.manager.name
"Guido"

# If the manager has changed in the database, when we refresh the band, the
# manager object will also be updated:
>>> await band.refresh()
>>> band.manager.name
"New name"

Also, improved the error messages when creating a BaseUser - thanks to @haaavk for this.