Django TypeError：预期的字符串或缓冲区

Question

I have an issue that sometimes pops up when I add a DateField to my models. I believe it to occur when I add a Date field to a table that already has data in. The problem is that i can not make the error go away. Every time i run migrate it errors TypeError: expected string or buffer

Is there a way to force the migrate. It even happens after I have deleted the field from the model, deleted the database and deleted migrations. I have also tried manually entering in the datestamp into the database.

The only way I can get around it is to create a new project and copying the code over.

I added creation and last_modified to the model that coursed the error

class Entry(models.Model):
    title = models.CharField(max_length=50, blank=True, null=True)
    start = models.DateField(blank=True, null=True)
    end = models.DateField(blank=True, null=True)

    creation = models.DateField(auto_now_add=True)
    last_modified = models.DateField(auto_now=True)

    def __unicode__(self):
        return self.title

I have since reverted back to when it was working.

class Entry(models.Model):
    title = models.CharField(max_length=50, blank=True, null=True)
    start = models.DateField(blank=True, null=True)
    end = models.DateField(blank=True, null=True)

    def __unicode__(self):
        return self.title

Full Error:

  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/core/management/base.py", line 393, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/core/management/base.py", line 444, in execute
    output = self.handle(*args, **options)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 222, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/migrations/executor.py", line 148, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/migrations/migration.py", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/migrations/operations/fields.py", line 62, in database_forwards
    field,
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/backends/sqlite3/schema.py", line 179, in add_field
    self._remake_table(model, create_fields=[field])
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/backends/sqlite3/schema.py", line 77, in _remake_table
    self.effective_default(field)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 211, in effective_default
    default = field.get_db_prep_save(default, self.connection)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 710, in get_db_prep_save
    prepared=False)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 1322, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 1317, in get_prep_value
    return self.to_python(value)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 1274, in to_python
    parsed = parse_date(value)
  File "/Users/User/pyProjects/projects/Folder/Env/lib/python2.7/site-packages/django/utils/dateparse.py", line 60, in parse_date
    match = date_re.match(value)
TypeError: expected string or buffer

Answer 1

auto_now and auto_now_add have caused problems with migrations in the past, the best approach would be to use the default field instead.

from django.utils import timezone

class Entry(models.Model):
    title = models.CharField(max_length=50, blank=True, null=True)
    start = models.DateField(blank=True, null=True)
    end = models.DateField(blank=True, null=True)

    creation = models.DateField(default=timezone.now())
    last_modified = models.DateField(default=timezone.now())

    def __unicode__(self):
        return self.title

I use the timezone module, this is preferred if your project is timezone aware.

Answer 2

If solution by @petkostas doesn't work try using str for your model instead of unicode

example:

def __str__(self):
        return self.title