Appending an item to a Django JsonField - Getting a TypeError

Question

In my models.py, I have the following code:

from __future__ import unicode_literals
from django.db import models
from django.contrib.postgres.fields import JSONField
import json

class Table(models.Model):
    name = models.CharField(max_length=255)
    structure = JSONField(default=json.dumps('{}'))

    def __unicode__(self):
        return self.name

class Column(models.Model):
    table = models.ForeignKey(Table, related_name='columns')
    name = models.CharField(max_length=255)
    required = models.BooleanField(default=True)

    def __unicode__(self):
        return self.name + ' FROM TABLE ' + self.table.name

    def save(self, *args, **kwargs):
        if not self.pk:
            self.table.structure[self.name] = {
                'required' : self.required,
            }

As you can see from the code, when a Column is saved, if the column's required field gets added to the structure of the Table. However, when I try saving a column from the admin panel, I get the following error:

TypeError at /admin/myapp/column/add/
'unicode' object does not support item assignment

I think the problem is with the default value of my structure field. I also tried the following:

structure = JSONField(default={})
structure = JSONField(default='{}')
structure = JSONField(default=dict)

Each time, I got the same error. Any help? Thanks.

score 5 · Accepted Answer · answered Jan 23 '17 at 08:40

According to the doc, if you want the default to be a dict the right definition is JSONField(default=dict). Note that you may need to clean up your existing database - what has been stored as a unicode string (with your original definition) will STILL be unserialized as a unicode string.

Not that this is very cleary documented:

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.

so you could have saved you some trouble by reading the doc first.

I actually did read the doc, but had a migration that set it to {} before. I didn't really figure the database clearing part. Thanks though. — darkhorse, Jan 23 '17 at 14:27

score -2 · Answer 2 · edited May 23 '17 at 11:45

-2

EDIT Just leave off the default value since it's empty. That is already the default. You could also use None as the default (see this post)

edited May 23 '17 at 11:45

Community

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answered Jan 23 '17 at 06:42

denvaar

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From the doc : "A field for storing JSON encoded data. In Python the data is represented in its Python native format: dictionaries, lists, strings, numbers, booleans and None" - so that's not the problem. – bruno desthuilliers Jan 23 '17 at 08:34
@brunodesthuilliers you're right -- I was mistaken for a different json field package. – denvaar Jan 23 '17 at 16:53

Appending an item to a Django JsonField - Getting a TypeError

2 Answers2