title: django.db.utils.IntegrityError: (1062, "Duplicate entry '' for key 'slug'")
body: <p>I'm trying to follow the tangowithdjango book and must add a slug to update the category table. However I'm getting an error after trying to  migrate the databases.</p>
  
  <p><a href="http://www.tangowithdjango.com/book17/chapters/models_templates.html#creating-a-details-page" rel="noreferrer">http://www.tangowithdjango.com/book17/chapters/models_templates.html#creating-a-details-page</a></p>
  
  <p>I didn't provide a default value for the slug, so Django asked me to provide one and as the book instructed I type in ''.</p>
  
  <p>It's worth noticing that instead of using sqlite as in the original book I'm using mysql.  </p>
  
  <pre><code>models.py
  from django.db import models
  from django.template.defaultfilters import slugify
  
  # Create your models here.
  class Category(models.Model):
        name = models.CharField(max_length=128, unique=True)
        views = models.IntegerField(default=0)
        likes = models.IntegerField(default=0)
        slug = models.SlugField(unique=True)
  
        def save(self, *args, **kwargs):
                self.slug = slugify(self.name)
                super(Category, self).save(*args, **kwargs)
  
         class Meta:
                verbose_name_plural = "Categories"
  
         def __unicode__(self):
                return self.name
  
  class Page(models.Model):
          category = models.ForeignKey(Category)
          title = models.CharField(max_length=128)
          url = models.URLField()
          views = models.IntegerField(default=0)
  
          def __unicode__(self):
                  return self.title
  </code></pre>
  
  <p>The command prompt</p>
  
  <pre><code>sudo python manage.py migrate       
  Operations to perform:
     Apply all migrations: admin, rango, contenttypes, auth, sessions
  Running migrations:
    Applying rango.0003_category_slug...Traceback (most recent call last):
    File "manage.py", line 10, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 385, in  execute_from_command_line
  utility.execute()
   File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 377, in execute
  self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 288, in run_from_argv
  self.execute(*args, **options.__dict__)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 338, in execute
  output = self.handle(*args, **options)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/migrate.py", line 160, in handle
  executor.migrate(targets, plan, fake=options.get("fake", False))
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 63, in migrate
  self.apply_migration(migration, fake=fake)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 97, in apply_migration
  migration.apply(project_state, schema_editor)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/migration.py", line 107, in apply
  operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 37, in database_forwards
  field,
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/mysql/schema.py", line 42, in add_field
  super(DatabaseSchemaEditor, self).add_field(model, field)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 411, in add_field
  self.execute(sql, params)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 98, in execute
  cursor.execute(sql, params)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/utils.py", line 81, in execute
  return super(CursorDebugWrapper, self).execute(sql, params)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/utils.py", line 65, in execute
  return self.cursor.execute(sql, params)
    File "/usr/local/lib/python2.7/dist-packages/django/db/utils.py", line 94, in __exit__
  six.reraise(dj_exc_type, dj_exc_value, traceback)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/utils.py", line 65, in execute
  return self.cursor.execute(sql, params)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/mysql/base.py", line 128, in execute
  return self.cursor.execute(query, args)
    File "/usr/local/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 205, in execute
  self.errorhandler(self, exc, value)
    File "/usr/local/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
  raise errorclass, errorvalue
  django.db.utils.IntegrityError: (1062, "Duplicate entry '' for key 'slug'")
  </code></pre>