django error on migration: "There is no unique constraint matching given keys for referenced table

Question

So I have seen that a lot of these kinds of questions have popped up (few answered) and none in a Django aspect that I saw. I am confused why I am getting the error, I am g

Answer 1

Codejoy,

When you define a primarykey, it is automatically set as unique.. So, just go by:

class ZoneEntity(models.Model):
    zone_number = models.CharField(max_length=100, primary_key=True)
    ....

class CesiumEntity(models.Model):
    ...
    zone_id = models.ForeignKey('ZoneEntity', null=True, blank=True)
    ...

This will automatically bind the PK of ZoneEntity with zone_id!

If the field you are trying to make the relation IS NOT the primary key, then you can add unique=True and to_field='foo'

 - python manage.py. makemigration
s
Migrations for 'module1':
  0002_auto_20170214_1503.py:
    - Create model CesiumEntity
    - Create model ZoneEntity
    - Add field zone_id to cesiumentity

 - python manage.py migrate
Operations to perform:
  Synchronize unmigrated apps: staticfiles, messages
  Apply all migrations: admin, contenttypes, module1, auth, sessions
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying module1.0002_auto_20170214_1503... OK

Answer 2

To solve this, needed to add the unique constraint on the postgres table id myself.

psql <your-database-name>
ALTER TABLE swsite_zoneentity ADD CONSTRAINT zone_unique_id UNIQUE(id);

Like this answer