Changing JSONField on inline in admin doesn't always trigger change

[john-parton]

If you have an inline with a JSONField and an initial value of {'key': 1} and you try to change it to {'key': True} without any other changes, the admin won't save your change. I believe this is because JSONField.has_changed() just uses simple equality checking and {'key': True} == {'key': 1} with a basic equality test in python.

Here's the minimal test case:

>>> from django.contrib.postgres.forms import JSONField
>>> JSONField().has_changed({'a': True}, '{"a": 1}')
False

Let me know if there's any other information you need.

Thanks.

[john-parton]

Here's some information that might be helpful: ​https://stackoverflow.com/questions/45888263/strict-comparison-of-dictionaries-in-python

It looks like there's two approaches:

1. Convert both the old value and new value to some string representation and compare them. (Either using json.dumps or pprint.pformat)
2. Recursively compare the values

I think (1) with json.dumps would probably work best. pprint.pformat was suggested to handle objects that can't be json-serialized, but obviously everything that can go into a JSONField can be json-serialized.

[shangdahao]

​PR

Hi, @john-partonm, for (1), json.dumps works well, but I think str() is a good choice too, so I use it.

[john-parton]

Replying to hui shang:

​PR

Hi, @john-partonm, for (1), json.dumps works well, but I think str() is a good choice too, so I use it.

Does str() always sort dictionary keys? I'm not totally sure, but under certain circumstances I think your fix might flag {'a': 1, 'b': 2} as being different from {'b': 2, 'a': 1}.

Thanks for the quick response and your hard work!

[shangdahao]

Replying to john-parton:

Replying to hui shang:

​PR

Hi, @john-partonm, for (1), json.dumps works well, but I think str() is a good choice too, so I use it.

Does str() always sort dictionary keys? I'm not totally sure, but under certain circumstances I think your fix might flag {'a': 1, 'b': 2} as being different from {'b': 2, 'a': 1}.

Thanks for the quick response and your hard work!

EDIT:

At least with Python3.6, the string representation of two dicts can differ:

  Python 3.6.1 (default, Dec 2015, 13:05:11)
  [GCC 4.8.2] on linux
   left = {}
   left['a'] = 1
   left['b'] = 2
   right = {}
   right['b'] = 2
   right['a'] = 1
str(left) == str(right)
=> False   

Yes, you are right, thank you for the explanation. I have changed the comparison from str() to json.dumps

[Tim Graham <timograham@…>]

In 1907fc9b:

Fixed #28534 -- Made JSONField.has_changed() ignore key order and consider True/1 values as different.