Definition of JSONField causes issue with validation for a Falsey value

[Adam Carruthers]

I ran into this issue while trying to create a JSONField that would contain a list of strings. The list of strings could have any length. I used the following:

model_field = models.JSONField(validator=[MyCustomValidator()])

The validator checks that the object is a list of strings along with some other stuff.

However, under this configuration, [] was not a valid entry because it was falsey and hence it failed the blank=False test.

So I set blank=True on the field to allow for []. But this led to the problem that if you have a falsey value then the validators aren't run. So I was able to set the field to be null on the admin site and it was permitted. But I really don't want to allow null as a value for fear of causing bugs in my code that uses the API. Even worse an empty string "" was allowed!

As it is, as far as I can tell, there is no way to create a field level validator that allows empty list and does not allow null. You could probably use a model level validator but that is non-ideal, and this feels like a potential way for bugs to emerge on a website.

[Mariusz Felisiak]

Yes, Django doesn't run validators on empty values, however you can create a custom form field and change empty_values to the ([],), see #19997 and ​an example in tests. Please use one of support channels if you have further questions.