问django.db.utils.IntegrityError:唯一约束失败: first_app_topic.top_name

EN

from django.db import models

# Create your models here.
class Topic(models.Model):
    top_name = models.CharField(max_length=264, unique=True)

    def __str__(self):
        return self.top_name

class Webpage(models.Model):
    topic = models.ForeignKey(Topic, on_delete=models.SET_NULL) # added on_delete parameter, because DJANGO > v2.0 is 
    name = models.CharField(max_length=264, unique=True)
    url = models.URLField(unique=True)

    def __str__(self):
        return self.name

class AccessRecord(models.Model):
    name = models.ForeignKey(Webpage, on_delete=models.SET_NULL) # same on_delete needed to be added otherwise t>python manage.py migrate fails
    date = models.DateField()

    def __str__(self):
        return str(self.date)

python manage.py makemigrations registers the changes to your application.
python manage.py migrate
python manage.py shell

>>> from first_app.models import Topic
>>> print(Topic.objects.all())
<QuerySet [Topic: Social Network]>
>>> t = Topic(top_name="Social Network")
>>> t.save()

Traceback (most recent call last):
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\sqlite3\base.py", line 396, in execute
    return Database.Cursor.execute(self, query, params)
sqlite3.IntegrityError: UNIQUE constraint failed: first_app_topic.top_name

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\base.py", line 746, in save
    force_update=force_update, update_fields=update_fields)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\base.py", line 784, in save_base
    force_update, using, update_fields,
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\base.py", line 887, in _save_table
    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\base.py", line 926, in _do_insert
    using=using, raw=raw,
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\query.py", line 1204, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\models\sql\compiler.py", line 1392, in execute_sql
    cursor.execute(sql, params)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 100, in execute
    return super().execute(sql, params)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
  File "C:\....\MyDjangoEnv\lib\site-packages\django\db\backends\sqlite3\base.py", line 396, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: UNIQUE constraint failed: first_app_topic.top_name

from django.contrib import admin
from first_app.models import AccessRecord, Topic, Webpage
# Register your models here.
admin.site.register(AccessRecord)
admin.site.register(Topic)
admin.site.register(Webpage)

python manage.py createsuperuser

SystemCheckError: System check identified some issues:

ERRORS:
first_app.AccessRecord.name: (fields.E320) Field specifies on_delete=SET_NULL, but cannot be null.
        HINT: Set null=True argument on the field, or change the on_delete rule.
first_app.Webpage.topic: (fields.E320) Field specifies on_delete=SET_NULL, but cannot be null.
        HINT: Set null=True argument on the field, or change the on_delete rule.