Failing python-unit tests: "AttributeError: 'NoneType' object has no attribute 'cluster'"

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ERROR: test_minimum_valid_pools (ceph.tests.CephPoolSerializerTest)
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Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/srv/openattic/backend/ceph/tests.py", line 418, in test_minimum_valid_pools
    self.assertTrue(s.is_valid(), 'pool={} errors={}'.format(pool, s.errors))
  File "/usr/lib/python2.7/site-packages/rest_framework/serializers.py", line 554, in is_valid
    return not self.errors
  File "/usr/lib/python2.7/site-packages/rest_framework/serializers.py", line 546, in errors
    ret = self.from_native(data, files)
  File "/usr/lib/python2.7/site-packages/rest_framework/serializers.py", line 1025, in from_native
    instance = super(ModelSerializer, self).from_native(data, files)
  File "/usr/lib/python2.7/site-packages/rest_framework/serializers.py", line 379, in from_native
    return self.restore_object(attrs, instance=getattr(self, 'object', None))
  File "/usr/lib/python2.7/site-packages/rest_framework/serializers.py", line 1003, in restore_object
    instance = self.opts.model()
  File "/srv/openattic/backend/ceph/models.py", line 336, in __init__
    self.cluster = CephPool.objects.nodb_context.cluster
AttributeError: 'NoneType' object has no attribute 'cluster'