django.db.utils.IntegrityError: column "venue_city" contains null values
<p>I have read a lot of other posts here on stackoverflow and google but I could not find a solution. </p>

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

<pre><code>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
</code></pre>

<p>My model is as follows:</p>

<pre><code>class Venue(models.Model):
    venue_city = models.ForeignKey(City, null=True,)
    venue_country=models.ForeignKey(Country, null=True)
</code></pre>

<p>venue_country did not exist before so that migration happened successfully. But venue_city was a CharField. </p>

<p>I made some changes to my migration file so that it would execute the sql as follows:</p>

<pre><code># -*- 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'''),
</code></pre>

<p>Thanks in advance!</p>

<p>UPDATE: my new migration file:</p>

<pre><code># -*- 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'),
        ),
    ]
</code></pre>