Updated NestedBoundField to also handle empty string when rendering its form

[Ernest0x]

If a NestedBoundField field has a value of None and is inside another NestedBoundField field, it will have its value converted to an empty string while the form of its enclosing field is being rendered. So, NestedBoundField fields with an empty string value must be handled the same way as NestedBoundField fields with a None value.

[lovelydinosaur]

Could you give a simple example, demonstrating exactly what gets rendered incorrectly, and how this fix affects it?

[Ernest0x]

Could you give a simple example, demonstrating exactly what gets rendered incorrectly, and how this fix affects it?

Of course. I have made the following #3464 (comment)

[lovelydinosaur]

Okay. That's something, tho a minimal code example would still be helpful.

[andrewdodd]

I agree, I'd love to see a basic example of how this occurs.

[Ernest0x]

Ok, here is code that reproduces exactly the error that this pull request fixes:

from rest_framework import serializers
from rest_framework.renderers import HTMLFormRenderer

class Level2NestedSerializer(serializers.Serializer):
    some_field = serializers.CharField()

class Level1NestedSerializer(serializers.Serializer):
    nested2_field = Level2NestedSerializer(allow_null=True)
    another_field = serializers.CharField()

class Level0NestedSerializer(serializers.Serializer):
    nested1_field = Level1NestedSerializer()

serializer = Level0NestedSerializer(data={
    'nested1_field': {
        'nested2_field': None,
        'another_field': 'test'
    }
})

serializer.is_valid()

renderer = HTMLFormRenderer()
for field in serializer:
    rendered = renderer.render_field(field, {})

[lovelydinosaur]

Okay, and how does the HTML form render before/after the fix. (Include screenshots if you're feeling generous! 😎 )

[Ernest0x]

Okay, and how does the HTML form render before/after the fix. (Include screenshots if you're feeling generous! )

Before the fix, it does not render at all. It crashes with a traceback.
After the fix, it renders ok. For the nested field with None value, an empty form is rendered.

[lovelydinosaur]

Would you consider pulling together a test case? Failing that we might consider merging in any case and adding that as an outstanding issue.

[Ernest0x]

Would you consider pulling together a test case?

Sure. Added in this pull request.

[lovelydinosaur]

Good stuff, thanks!