Django 1.9 JSONField update behavior

Question

I've recently updated to Django 1.9 and tried updating some of my model fields to use the built-in JSONField (I'm using PostgreSQL 9.4.5). As I was trying to create and update my object's fields, I came across something peculiar. Here is my model:

class Activity(models.Model):
    activity_id = models.CharField(max_length=MAX_URL_LENGTH, db_index=True, unique=True)
    my_data = JSONField(default=dict())

Here is an example of what I was doing:

>>> from proj import models
>>> test, created = models.Activity.objects.get_or_create(activity_id="foo")
>>> created
True
>>> test.my_data['id'] = "foo"
>>> test.save()
>>> test
<Activity: {"id": "foo"}>
>>> test2, created2 = models.Activity.objects.get_or_create(activity_id="bar")
>>> created2
True
>>> test2
<Activity: {"id": "foo"}>
>>> test2.activity_id
'bar'
>>> test.activity_id
'foo'

It seems whenever I update any field in my_data, the next object I create pre-populates itself with the data from my_data from the previous object. This happens whether I use get_or_create or just create. Can someone explain to me what is happening?

Answer 1

The problem is that you are using default=dict(). Python dictionaries are mutable. The default dictionary is created once when the models file is loaded. After that, any changes to instance.my_data alter the same instance, if they are using the default value.

The solution is to use the callable dict as the default instead of dict().

class Activity(models.Model):
    my_data = JSONField(default=dict)

The JSONField docs warn about this:

If you give the field a default, ensure it’s a callable such as dict (for an empty default) or a callable that returns a dict (such as a function). Incorrectly using default={} creates a mutable default that is shared between all instances of JSONField.