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I have read a lot of other posts here on stackoverflow and google but I could not find a solution.

It all started when I changed the model from a CharField to a ForeignKey. The error I recieve is:

Operations to perform:
  Synchronize unmigrated apps: gis, staticfiles, crispy_forms, geoposition, messages
  Apply all migrations: venues, images, amenities, cities_light, registration, auth, admin, sites, sessions, contenttypes, easy_thumbnails, newsletter
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying venues.0016_auto_20160514_2141...Traceback (most recent call last):
  File "/Users/iam-tony/.envs/venuepark/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
psycopg2.IntegrityError: column "venue_city" contains null values
My model is as follows:
class Venue(models.Model):
    venue_city = models.ForeignKey(City, null=True,)
    venue_country=models.ForeignKey(Country, null=True)
venue_country did not exist before so that migration happened successfully. But venue_city was a CharField. 
I made some changes to my migration file so that it would execute the sql as follows:
# -*- coding: utf-8 -*-
from __future__ import unicode_literals
from django.db import migrations, models
class Migration(migrations.Migration):
    dependencies = [
        ('venues', '0011_venue_map_activation'),
    migrations.RunSQL(''' ALTER TABLE venues_venue ALTER venue_city TYPE integer USING  venue_city::integer '''),
    migrations.RunSQL(''' ALTER TABLE venues_venue ALTER venue_city RENAME COLUMN venue_city TO venue_city_id '''),
    migrations.RunSQL(''' ALTER TABLE venues_venue ADD CONSTRAINT venues_venus_somefk FOREIGN KEY (venue_city_id) REFERENCES  cities_light (id) DEFERRABLE INITIALLY DEFERRED'''),
Thanks in advance!
UPDATE: my new migration file:
# -*- coding: utf-8 -*-
from __future__ import unicode_literals
from django.db import migrations, models
class Migration(migrations.Migration):
    dependencies = [
        ('cities_light', '0006_compensate_for_0003_bytestring_bug'),
        ('venues', '0024_remove_venue_venue_city'),
    operations = [
        migrations.AddField(
            model_name='venue',
            name='venue_city',
            field=models.ForeignKey(null=True, to='cities_light.City'),
                I think, you added null=True after created migration file. Because venue_city  is not a nullable field in your migration file
– Anoop
                May 15, 2016 at 19:23
Looks like you added null=True after created migration file. Because venue_city is not a nullable field in your migration file
Follow these steps.
1) Drop venue_city & venue_country from your local table
3) Delete all the migration files you created for these `CharField to a ForeignKey` change
4) execute `python manage.py makemigrations`
5) execute 'python manage.py migrate'
It should work
                Just as a quick comment, if you are using something like AWS Elastic Beanstalk it could be easier to simply correct the models, delete the erroneous migrations and makemigrations again.
– reojased
                Jan 23, 2020 at 5:35
                :-( my help ends there.But because migration files are Dependant with previous files.Try inspecting them because the error is a result of conflicting commits to the database
– 0n10n_
                May 15, 2016 at 20:10
add null=True, blank=True in Venue model class
eg: venue_city = models.ForeignKey(City, null=True, blank=True)
delete venues.0016_auto_20160514_2141 migration file from migrations folder in your app
then run python manage.py makemigrations
then run python manage.py migrate
no need to drop table or remove migrations from migration tables
First delete last migration that face problem
Then add like venue_city = models.CharField(blank=True, null=True)
Finally use makemigrations and migrate command
As the error says, you have a null value in the "venue_city" column.
It seems at the time of defining your model you had not included "null=True".
So it returns null because there is nothing in your 'venue_city" variable. Consider deleting the last migration file and perform another migration using
python manage.py makemigrations and python manage.py migrate commands.
        
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