title: Django 1.7 ValueError: invalid literal for int() with base 10: 'a'
body: <p>I am getting this error:</p>
  
  <pre><code>    Operations to perform:
    Apply all migrations: account, jobs, assets, sessions, admin, auth, laptops, contenttypes, mardes
  Running migrations:
    Applying assets.0004_auto_20150202_1707...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 397, in add_field
      definition, params = self.column_sql(model, field, include_default=True)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 120, in column_sql
      default_value = self.effective_default(field)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 183, in effective_default
      default = field.get_db_prep_save(default, self.connection)
    File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 1722, in get_db_prep_save
      return self.related_field.get_db_prep_save(value, connection=connection)
    File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 627, in get_db_prep_save
      prepared=False)
    File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 907, in get_db_prep_value
      value = self.get_prep_value(value)
    File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 915, in get_prep_value
      return int(value)
  ValueError: invalid literal for int() with base 10: 'a'
  </code></pre>
  
  <p>When running python manage.py migrate.</p>
  
  <p>For this app there is only an admin.py and models.py file, everything else is blank.  Here is the models.py file:</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import User
  
  class Asset_type(models.Model):
      name = models.CharField(max_length=50)
  
      def __str__(self):
          return self.name
  
  class Asset_os(models.Model):
      name = models.CharField(max_length=50)
  
      def __str__(self):
          return self.name
  
  class Asset_cs_status(models.Model):
      status = models.CharField(max_length=50)
  
      def __str__(self):
          return self.status
  
  class Asset_floor(models.Model):
      name = models.CharField(max_length=50)
  
      def __str__(self):
          return self.name
  
  class Asset(models.Model):
      DLO = (
              ('0', 'Desktop'),
              ('1', 'Laptop'),
              ('2', 'Other'),
              )
      user = models.ForeignKey(User)
      asset_type = models.ForeignKey(Asset_type)
      asset_os = models.ForeignKey(Asset_os)
      asset_cs_status = models.ForeignKey(Asset_cs_status)
      asset_floor = models.ForeignKey(Asset_floor)
      ppl = models.CharField(max_length=40, unique=True)
      desktop_laptop = models.CharField(max_length=1, choices=DLO) 
      date_of_purchase = models.DateField()
      extra_info = models.CharField(max_length=250, default='-')
      pc_name = models.CharField(max_length=100, null=True, default='-')
      bookable = models.BooleanField(default=False)
      image = models.ImageField(upload_to='asset_files/%Y/%m/%d', null=True, blank=True, help_text='Optional')
  
      def __str__(self):
          return self.asset_type.name+' ('+self.ppl+')' 
  </code></pre>
  
  <p>Please let me know if you need to see anything else.
  BC</p>