python - Django - UNIQUE 制約が失敗しました: auth_user.username

Question

Django REST Frameworkアプリ の単体テストfactory_boyを作成し、偽のテスト データを作成するために使用しています。テストを実行すると、次のエラーが発生します。

File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 254, in _find_tests
  module = self._get_module_from_name(name)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
  __import__(name)
File "/Users/thomasheatwole/osf-meetings/meetings/conferences/tests.py", line 27, in <module>
  class SubmissionFactory(factory.DjangoModelFactory):
File "/Users/thomasheatwole/osf-meetings/meetings/conferences/tests.py", line 34, in SubmissionFactory
  username = 'contributor'
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/factory/base.py", line 67, in __call__
  return cls.create(**kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/factory/base.py", line 594, in create
  return cls._generate(True, attrs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/factory/base.py", line 519, in _generate
  obj = cls._prepare(create, **attrs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/factory/base.py", line 494, in _prepare
  return cls._create(model_class, *args, **kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/factory/django.py", line 181, in _create
  return manager.create(*args, **kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/manager.py", line 122, in manager_method
  return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/query.py", line 401, in create
  obj.save(force_insert=True, using=self.db)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/contrib/auth/base_user.py", line 74, in save
  super(AbstractBaseUser, self).save(*args, **kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/base.py", line 708, in save
  force_update=force_update, update_fields=update_fields)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/base.py", line 736, in save_base
  updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/base.py", line 820, in _save_table
  result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/base.py", line 859, in _do_insert
  using=using, raw=raw)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/manager.py", line 122, in manager_method
  return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/query.py", line 1039, in _insert
  return query.get_compiler(using=using).execute_sql(return_id)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 1060, in execute_sql
  cursor.execute(sql, params)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
  return self.cursor.execute(sql, params)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/utils.py", line 95, in __exit__
  six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
  return self.cursor.execute(sql, params)
File "/Users/thomasheatwole/.virtualenvs/django/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 323, in execute
  return Database.Cursor.execute(self, query, params)
IntegrityError: UNIQUE constraint failed: auth_user.username

それが何を意味するのか、何が原因なのかはよくわかりません。ファクトリが定義されている場所は次のとおりです。

class UserFactory(factory.DjangoModelFactory):
    class Meta: 
        model = User

インスタンスが作成される場所は次のとおりです。

contributor = UserFactory(
    username = 'contributor'
    )

完全なファイルは次のとおりです: tests.py

簡単な修正方法があれば教えてください。ありがとう！