问ResetDjango postgresql数据库？刷新不起作用

EN

python3 manage.py makemigrations
python3 manage.py migrate

vagrant@vagrant-ubuntu-trusty-64:/vagrant/grader$ python3 manage.py makemigrations
No changes detected
vagrant@vagrant-ubuntu-trusty-64:/vagrant/grader$ python3 manage.py migrate     Operations to perform:
  Apply all migrations: contenttypes, sessions, admin, auth, core
Running migrations:
  Rendering model states... DONE
  Applying core.0002_auto_20160103_0955...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/__init__.py", line 342, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/commands/migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/executor.py", line 92, in migrate
    self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/executor.py", line 198, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/migration.py", line 123, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/operations/fields.py", line 62, in database_forwards
    field,
  File "/usr/local/lib/python3.4/dist-packages/django/db/backends/base/schema.py", line 382, in add_field
    definition, params = self.column_sql(model, field, include_default=True)
  File "/usr/local/lib/python3.4/dist-packages/django/db/backends/base/schema.py", line 145, in column_sql
    default_value = self.effective_default(field)
  File "/usr/local/lib/python3.4/dist-packages/django/db/backends/base/schema.py", line 210, in effective_default
    default = field.get_db_prep_save(default, self.connection)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/fields/related.py", line 910, in get_db_prep_save
    return self.target_field.get_db_prep_save(value, connection=connection)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/fields/__init__.py", line 728, in get_db_prep_save
    prepared=False)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/fields/__init__.py", line 968, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/fields/__init__.py", line 976, in get_prep_value
    return int(value)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'datetime.datetime'

python3 manage.py flush  
python3 manage.py makemigrations
python3 manage.py migrate

vagrant@vagrant-ubuntu-trusty-64:/vagrant/grader$ python3 manage.py flush
You have requested a flush of the database.
This will IRREVERSIBLY DESTROY all data currently in the 'graderdb' database,
and return each table to an empty state.
Are you sure you want to do this?

    Type 'yes' to continue, or 'no' to cancel: yes
CommandError: Database graderdb couldn't be flushed. Possible reasons:
  * The database isn't running or isn't configured correctly.
  * At least one of the expected database tables doesn't exist.
  * The SQL was invalid.
Hint: Look at the output of 'django-admin sqlflush'. That's the SQL this command wasn't able to run.
The full error: cannot truncate a table referenced in a foreign key constraint
DETAIL:  Table "core_mark" references "core_student".
HINT:  Truncate table "core_mark" at the same time, or use TRUNCATE ... CASCADE.