django - UNIQUE CONSTRAINED FAILED error
Question: This is what my models.py looks like:

    
    
       from django.db import models
       from django.core.validators import RegexValidator
    
        # Create your models here.
        class Customer(models.Model):
            customer_id = models.AutoField(primary_key=True,unique=True)
            full_name = models.CharField(max_length=50)
            user_email = models.EmailField(max_length=50)
            user_pass = models.CharField(max_length=30)
    
            def __str__(self):
                return "%s" % self.full_name
    
        class CustomerDetail(models.Model):
            phone_regex = RegexValidator(regex = r'^\d{10}$', message = "Invalid format! E.g. 4088385778")
            date_regex = RegexValidator(regex = r'\d{2}[-/]\d{2}[-/]\d{2}', message = "Invalid format! E.g. 05/16/91")
    
            address = models.CharField(max_length=100)
            date_of_birth = models.CharField(validators = [date_regex], max_length = 10, blank = True)
            company = models.CharField(max_length=30)
            home_phone = models.CharField(validators = [phone_regex], max_length = 10, blank = True)
            work_phone = models.CharField(validators = [phone_regex], max_length = 10, blank = True)
            customer_id = models.ForeignKey(Customer, on_delete=models.CASCADE)
    

I added `customer_id` to `Customer` after I added the same in `CustomerDetail`
as foreign key. Why do I still get this error after running migrate, even
after I added `unique=True` to customer_id?

    
    
    Error:
          Rendering model states... DONE
          Applying newuser.0003_auto_20160823_0128...Traceback (most recent call last):
              File "/home/krag91/Documents/djangodev/virtualenv    /lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in     execute
      return self.cursor.execute(sql, params)
              File "/home/krag91/Documents/djangodev/virtualenv    /lib/python3.5/site-packages/django/db/backends/sqlite3/base.py", line 337, in execute
      return Database.Cursor.execute(self, query, params)
          sqlite3.IntegrityError: UNIQUE constraint failed:     newuser_customer.customer_id
    The above exception was the direct cause of the following exception:
    
    Traceback (most recent call last):
      File "manage.py", line 22, in <module>
        execute_from_command_line(sys.argv)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/core/management/__init__.py", line 367, in execute_from_command_line utility.execute()
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/core/management/__init__.py", line 359, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/core/management/base.py", line 305, in run_from_argv
        self.execute(*args, **cmd_options)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/core/management/base.py", line 356, in execute
        output = self.handle(*args, **options)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/core/management/commands/migrate.py", line 202, in handle
        targets, plan, fake=fake, fake_initial=fake_initial
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/migrations/executor.py", line 97, in migrate
        state = self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/migrations/executor.py", line 132, in _migrate_all_forwards
        state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/migrations/executor.py", line 237, in apply_migration
        state = migration.apply(state, schema_editor)
      File "/home/krag91/Documents/djangodev/virtualenv/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/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/migrations/operations/fields.py", line 84, in database_forwards
        field,
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/sqlite3/schema.py", line 231, in add_field
        self._remake_table(model, create_fields=[field])
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/sqlite3/schema.py", line 199, in _remake_table
        self.quote_name(model._meta.db_table),
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 112, in execute
        cursor.execute(sql, params)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/utils.py", line 79, in execute
        return super(CursorDebugWrapper, self).execute(sql, params)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
        return self.cursor.execute(sql, params)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/utils.py", line 94, in __exit__
        six.reraise(dj_exc_type, dj_exc_value, traceback)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
        raise value.with_traceback(tb)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
        return self.cursor.execute(sql, params)
      File "/home/krag91/Documents/djangodev/virtualenv/lib/python3.5/site-packages/django/db/backends/sqlite3/base.py", line 337, in execute
        return Database.Cursor.execute(self, query, params)
    django.db.utils.IntegrityError: UNIQUE constraint failed: newuser_customer.customer_id
    


Answer: It seems like you are `already having some objects as per old model
definitions`. `By default django creates a field named as id to every model in
database`. It can be accesses by modelName.id.

In your case I guess what happened is that you are having some objects in
database with customer.id as primary as primary key. So when you changed the
models and applied migrations, `existing objects are checked and another
unique field is tried to added as a primary key`. Here the workaround is to
delete all the existing objects after removing the customer_id field and then
try recreating the field and run migrations.

HTH

