title: Django Migrations [ AttributeError: 'Startup' object has no attribute 'tags']
body: <p>This is my model and migration file. Its from The book <a href="https://django-unleashed.com/" rel="nofollow noreferrer">Django-Unleashed</a> - Chapter 10 . </p>
  
  <p><em>Models.py:</em></p>
  
  <pre><code>class Tag(models.Model):
      def get_update_url(self):
          return reverse('organizer_tag_update',
                         kwargs={'slug': self.slug})
      def get_absolute_url(self):
          return reverse('organizer_tag_detail',kwargs={'slug':self.slug})
      def get_delete_url(self):
          return reverse('organizer_tag_delete',
                         kwargs={'slug': self.slug})
  
      name = models.CharField(
          max_length=31, unique=True)
      slug = models.SlugField(
          max_length=31,
          unique=True,
          help_text='A label for URL config.'
          )
      class Meta:
          ordering=['name']
  
      def __str__(self):
          return self.name
  
  class Startup(models.Model):
  
      class Meta:
          ordering=['name']
          get_latest_by=['founded_date']
  
      name = models.CharField(
          max_length=31, db_index=True)
      slug = models.SlugField(
          max_length=31,
          unique=True,
          help_text='A label for URL config.')
  
      description = models.TextField()
      founded_date = models.DateField('date founded')
      contact = models.EmailField()
      website = models.URLField(max_length=255)
      tags = models.ManyToManyField(Tag,blank=True)
  
      def __str__(self):
          return self.name
      def get_update_url(self):
           return reverse('organizer_startup_update',kwargs={'slug': self.slug})
      def get_delete_url(self):
           return reverse('organizer_startup_delete',kwargs={'slug': self.slug})
      def get_absolute_url(self):
          return reverse('organizer_startup_detail',kwargs={'slug': self.slug})
  </code></pre>
  
  <p><strong>Migrations.py :</strong> </p>
  
  <pre><code>#startup_data.py        SECOND MIGRATION FILE
  
  from datetime import date
  from django.db import migrations, models
  
  STARTUPS = [
      {
          "name": "Arachnobots",
          "slug": "arachnobots",
          "contact": "contact@arachnobots.com",
          "description":
              "Remote-controlled internet-enabled "
              "Spider Robots.",
          "founded_date": date(2014, 10, 31),
          "tags": ["mobile", "augmented-reality"],
          "website":
              "http://frightenyourroommate.com/",
      },
      {
          "name": "Boundless Software",
          "slug": "boundless-software",
          "contact": "hello@boundless.com",
          "description": "The sky was the limit.",
          "founded_date": date(2013, 5, 15),
          "tags": ["big-data"],
          "website": "http://boundless.com/",
      },
  ]
  
  def add_startup_data(apps, schema_editor):
      Startup = apps.get_model(
          'organiser', 'Startup')
      Tag = apps.get_model('organiser', 'Tag')
      for startup in STARTUPS:
          startup_object = Startup.objects.create(
              name=startup['name'],
              slug=startup['slug'],
              contact=startup['contact'],
              description=startup['description'],
              founded_date=startup['founded_date'],
              website=startup['website'])
          for tag_slug in startup['tags']:
              startup_object.tags.add(
                  Tag.objects.get(
                      slug=tag_slug))
  def remove_startup_data(apps, schema_editor):
      Startup = apps.get_model(
          'organiser', 'Startup')
      for startup in STARTUPS:
          startup_object = Startup.objects.get(
              slug=startup['slug'])
          startup_object.delete()
  class Migration(migrations.Migration):
       dependencies = [
          ('organiser', '0002_tag_data'),
      ]
       operations = [
          migrations.RunPython(
              add_startup_data,
              remove_startup_data)
      ]
  
  #tagdata.py      FIRST MIGRATION FILE
  
  TAGS = (
      # ( tag name, tag slug ),
      ("augmented reality", "augmented-reality"),
      ("big data", "big-data"),
      ("django", "django"),
      ("education", "education"),
      ("ipython", "ipython"),
      ("javascript", "javascript"),
      ("jupyter", "jupyter"),
      ("mobile", "mobile"),
      ("node.js", "node-js"),
      ("php", "php"),
      ("python", "python"),
      ("ruby on rails", "ruby-on-rails"),
      ("ruby", "ruby"),
      ("video games", "video-games"),
      ("web", "web"),
      ("zend", "zend"),
  )
  
  def add_tag_data(apps,schema_editor):
      Tag=apps.get_model('organiser','Tag')
      for tag_name,tag_slug in TAGS:
          Tag.objects.create(
          name=tag_name,
          slug=tag_slug
          )
  def remove_tag_data(apps,schema_editor):
      Tag=apps.get_model('organiser','Tag')
      for _,tag_slug in TAGS:
          tag=Tag.objects.get(slug=tag_slug)
          tag.delete()
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('organiser', '0001_initial'),
      ]
  
      operations = [
      migrations.RunPython(
                  add_tag_data,
                  remove_tag_data )
      ]
  </code></pre>
  
  <p>Full Error :</p>
  
  <blockquote>
    <p>migrations\0003_startup_data.py", line 1 13, in add_startup_data
        startup_object.tags.add( AttributeError: 'Startup' object has no attribute 'tags'</p>
  </blockquote>
  
  <p>The error appears when i try to run <code>migrate</code> command.</p>