commission fails with 500 error for /MAAS/metadata/latest/by-id/ and django.db.utils.IntegrityError

[John George]

Commission of nodes fails.

Reproduce by installing 2.0.0~beta6 and trigger commission of a machine.

||/ Name Version Architecture Description
+++-===============================-====================================-============-=================================================
ii maas 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all "Metal as a Service" is a physical cloud and IPAM
ii maas-cli 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS client and command-line interface
un maas-cluster-controller <none> <none> (no description available)
ii maas-common 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS server common files
ii maas-dhcp 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS DHCP server
ii maas-dns 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS DNS server
ii maas-proxy 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS Caching Proxy
ii maas-rack-controller 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all Rack Controller for MAAS
ii maas-region-api 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all Region controller API service for MAAS
ii maas-region-controller 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all Region Controller for MAAS
un maas-region-controller-min <none> <none> (no description available)
un python-django-maas <none> <none> (no description available)
un python-maas-client <none> <none> (no description available)
un python-maas-provisioningserver <none> <none> (no description available)
ii python3-django-maas 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS server Django web framework (Python 3)
ii python3-maas-client 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 2.0.0~beta6+bzr5073-0ubuntu1~xenial1 all MAAS server provisioning libraries (Python 3)

Trace from regiond.log:

2016-06-06 19:04:06 [HTTPChannel,222,127.0.0.1] 500 Error - /MAAS/metadata/latest/by-id/4y3h7y/
        Traceback (most recent call last):
          File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 496, in run
            appIterator = self.application(self.environ, self.startResponse)
          File "/usr/lib/python3/dist-packages/django/core/handlers/wsgi.py", line 189, in __call__
            response = self.get_response(request)
          File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 227, in get_response
            response = get_response(request)
          File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 216, in get_response
            request, get_resolver(None), sys.exc_info())
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 201, in get_response
            return django_get_response(request)
          File "/usr/lib/python3/dist-packages/django/db/transaction.py", line 225, in __exit__
            connection.commit()
          File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 173, in commit
            self._commit()
          File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 142, in _commit
            return self.connection.commit()
          File "/usr/lib/python3/dist-packages/django/db/utils.py", line 98, in __exit__
            six.reraise(dj_exc_type, dj_exc_value, traceback)
          File "/usr/lib/python3/dist-packages/django/utils/six.py", line 685, in reraise
            raise value.with_traceback(tb)
          File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 142, in _commit
            return self.connection.commit()
        django.db.utils.IntegrityError: insert or update on table "piston3_consumer" violates foreign key constraint "piston3_consumer_user_id_279f98de55b85c90_fk_auth_user_id"
        DETAIL: Key (user_id)=(2) is not present in table "auth_user".