Django JSONField not receiving cleaned data - django

Question

I have a very simple model which contains a JSONField:
class Thing(models.Model):
title = models.CharField(max_length=1024)
text = JSONField(default=dict)
I've created a custom widget that allows for the input of key-value pairs:
class JsonWidget(forms.widgets.Widget):
template_name = 'json_widget.html'
def get_context(self, name, value, attrs):
context = super().get_context(name, value, attrs)
data = json.loads(value)
if not data:
data = JSON_DEFAULT
context['data'] = data.items()
return context
def value_from_datadict(self, data, files, name):
keys = data.getlist('json-key')
values = data.getlist('json-value')
json_data = {k: v for k, v in zip(keys, values)}
return json_data
I coerce the dict returned by the widget into a string in the field's clean function on the form:
class ThingForm(forms.ModelForm):
class Meta:
model = Thing
fields = ['title', 'text']
widgets = {
'text': JsonWidget(),
}
def clean_text(self):
text = self.cleaned_data.get('text')
return json.dumps(text)
I've inspected the output of JsonWidget.value_from_datadict (dict) and ThingForm.clean_text (str) are the expected types. But when the object goes to save it throws an exception:
TypeError: the JSON object must be str, bytes or bytearray, not 'dict'
This is my first time building a custom widget for Django 1.11, is there something obvious I've missed here?
Thanks!

Answer 1

This was a tricky one but I eventually traced the problem back to Instance construction fails on form data check
While the JSONField is called text on the model, there's no matching field on the ModelForm. Instead, the key/value pairs are compiled & turned into a dict by the widget's value_from_datadict. However, the cleaned_data values are saved back to the instance only if the field name exists in the form's POST data. This caused the widget to throw an error because text could not be blank, and the TypeError exception was raised during the form re-render.
The workaround was to add a hidden input called text to the field and only use it to bypass the field name check during instantion.