Attributes being deleted on .save() about blitzdb HOT 8 OPEN

>>> r
Record({})
>>> r.__dict__                                                                            
{'_autoload': True, 'pk': '564a7c5dac00f40300a324ab', '_default_backend': <blitzdb.backends.file.backend.Backend object at 0x7f109d5a9320>, '_attributes': {}, '_lazy': True, 'embed': False}
>>> r.pk
>>> r.attributes
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/fiatjaf/comp/hack-json/venv/lib/python3.4/site-packages/blitzdb/document.py", line 164, in __getattribute__
    self.revert()
  File "/home/fiatjaf/comp/hack-json/venv/lib/python3.4/site-packages/blitzdb/document.py", line 424, in revert
    raise self.DoesNotExist("No primary key given!")
blitzdb.document.DoesNotExist: DoesNotExist(Record)

from blitzdb import Document
from blitzdb import FileBackend

class Sheet(Document):
    class Meta(Document.Meta):
        primary_key = '_id'

class Record(Document):
    class Meta(Document.Meta):
        primary_key = '_id'

db = FileBackend('/tmp/t.db')

sheet = Sheet({
    '_id': '3i24i3l',
    'something': 'papapa'
})

db.save(sheet)
db.commit()

sheet = db.get(Sheet, {'_id': '3i24i3l'})

sheet.records = []
for i in range(18):
    record = Record({
        '_id': '%s' % i,
        'prop': 'parara'
    })
    sheet.records.append(record)
    record.sheets = [sheet]

db.save(sheet)
db.save(record)
db.commit()

# ~

for s in db.filter(Sheet, {}):
    print('sheet: ', s)
    print('sheet: ', s.__dict__)
    print('sheet: ', s.pk)
    print('sheet: ', s.attributes)
    for r in s.records:
        print('record: ', r)
        print('record: ', r.__dict__)
        print('record: ', r.pk)
        print('record: ', r.attributes)

        r.pou = 'pôu'
        db.save(r)

Traceback (most recent call last):
  File "iso.py", line 48, in <module>
    print('record: ', r.attributes)
  File "/home/fiatjaf/comp/hack-json/venv/lib/python3.4/site-packages/blitzdb/document.py", line 164, in __getattribute__
    self.revert()
  File "/home/fiatjaf/comp/hack-json/venv/lib/python3.4/site-packages/blitzdb/document.py", line 425, in revert
    obj = self._default_backend.get(self.__class__, {self.get_pk_name(): self.pk})
  File "/home/fiatjaf/comp/hack-json/venv/lib/python3.4/site-packages/blitzdb/backends/file/backend.py", line 508, in get
    raise cls.DoesNotExist
blitzdb.document.DoesNotExist: DoesNotExist(Record)