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

EN

from django.db import models
from django.utils import timezone


class BlogPost(models.Model):
    author = models.ForeignKey('auth.User')
    title = models.CharField(max_length=100,unique=True)
    text = models.TextField()
    created_date = models.DateTimeField(default=timezone.now)
    published_date = models.DateTimeField(blank=True,null=True)

def publish(self):
    self.published_date=timezone.now()
    self.save()

def __str__(self):
    return self.title

  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages   /django/db/migrations/migration.py", line 129, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/migrations/operations/fields.py", line 216, in database_forwards
schema_editor.alter_field(from_model, from_field, to_field)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 515, in alter_field
old_db_params, new_db_params, strict)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/sqlite3/schema.py", line 262, in _alter_field
self._remake_table(model, alter_field=(old_field, new_field))
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/sqlite3/schema.py", line 206, in _remake_table
self.quote_name(model._meta.db_table),
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 120, in execute
cursor.execute(sql, params)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/utils.py", line 80, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/utils.py", line 94, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
  File "/home/letsperf/.conda/envs/MyDjangoEnv/lib/python3.5/site-packages/django/db/backends/sqlite3/base.py", line 328, in execute
return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: UNIQUE constraint failed: social_blogpost.title