Add default value to generated enum if not included in choices

[mvanlonden]

Closes #474

Django fields with default="something" may not be included in choices but the value is a valid initial value for the field. This adds an DEFAULT option to the enum generated if the default value is not present in the choices.

https://simpleisbetterthancomplex.com/tips/2016/07/25/django-tip-8-blank-or-null.html

[coveralls]

Coverage increased (+0.02%) to 94.381% when pulling 4732b34 on mvanlonden:blank-fields into ea2cd98 on graphql-python:master.

[zbyte64]

I think a person should be able to define the label for the blank option. For django forms the pattern is that you explicitly define a blank choice or pass empty_label: https://stackoverflow.com/questions/14541074/empty-label-choicefield-django

Another concern is how this gets squared with the proposal of letting people explicitly define enums.

[mvanlonden]

Ah didn't realize defining an empty option is more idiomatic. That makes sense

[mvanlonden]

@zbyte64 @firaskafri I've taken an alternative approach by inspecting the default value for a field. Often times a CharField will define "" as the default value and it is not included in the choices. This adds a DEFAULT value to the generated enum.

[danpalmer]

If I'm not mistaken (please say if I am!), I think choices is validate in the default field clean method, so unless the default value is in the choices, it's not a valid input. When we provide field choices as an enum, we're doing that to communicate the valid options in the type system, and allowing clients to depend on type safety to some extent.

I think that the choice enum should match what is valid for the field. I'd suggest that this problem could be solved by either providing the default value in a side-channel of some sort, perhaps elsewhere in the API, or alternatively, the default value could be added to the field choices. I don't think either of these solutions would be the responsibility of Graphene though.

What do you think? I'm open to other solutions!

[mvanlonden]

Yes @danpalmer after further investigation it looks like you're right. Previously wasn't able to find examples where an empty value was included in choices. https://github.com/django/django/blob/1123f4551158b7fc65d3bd88c375a4517dcd0720/tests/forms_tests/models.py#L37