error 500 when responsible user is selected when updating object

Detailed bug description:

Error 500 when responsible user is selected when updating object.

Steps to reproduce:

Update object when responsible user is created.

Expected results:

Update works.

Actual result:

It fails with 500 error:

Internal Server Error: /56/edit
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/utils/decorators.py", line 29, in _wrapper
    return bound_func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/utils/decorators.py", line 25, in bound_func
    return func(self, *args2, **kwargs2)
  File "./inventory/views/crud.py", line 37, in dispatch
    return super(CRUDMixin, self).dispatch(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/base.py", line 87, in dispatch
    return handler(request, *args, **kwargs)
  File "./inventory/views/crud.py", line 254, in post
    obj = form.save()
  File "/usr/lib/python2.7/dist-packages/django/forms/models.py", line 446, in save
    construct=False)
  File "/usr/lib/python2.7/dist-packages/django/forms/models.py", line 100, in save_instance
    save_m2m()
  File "/usr/lib/python2.7/dist-packages/django/forms/models.py", line 96, in save_m2m
    f.save_form_data(instance, cleaned_data[f.name])
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 1527, in save_form_da$
a
    setattr(instance, self.attname, data)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 840, in __set__
    manager.add(*value)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 581, in add
    self._add_items(self.source_field_name, self.target_field_name, *objs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 670, in _add_items
    for obj_id in new_ids
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 356, in bulk_create
    self._batched_insert(objs_without_pk, fields, batch_size)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 835, in _batched_insert
    using=self.db)
 File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 232, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 1511, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 898, in execute_sql
    cursor.execute(sql, params)
  File "/usr/lib/python2.7/dist-packages/django/db/backends/util.py", line 53, in execute
    return self.cursor.execute(sql, params)
  File "/usr/lib/python2.7/dist-packages/django/db/utils.py", line 99, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/usr/lib/python2.7/dist-packages/django/db/backends/util.py", line 53, in execute
    return self.cursor.execute(sql, params)
  File "/usr/lib/python2.7/dist-packages/django/db/backends/mysql/base.py", line 124, in execute
    return self.cursor.execute(query, args)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
IntegrityError: (1452, 'Cannot add or update a child row: a foreign key constraint fails (`racks`.`invent
ory_inventoryobject_responsible`, CONSTRAINT `racksuser_id_refs_id_1f3b0e6e8e82e9a4` FOREIGN KEY (`racksu
ser_id`) REFERENCES `auth_user` (`id`))')

Reproducibility:

Always.

Workaround:

Not adding responsible user.

Impact:

It hurts.

Description of the environment:
https://inventory.infra.mirantis.net/
python-django-racks 0.1+git123-gc88afb1