WTForms-JSON with optional nesting using FormField
wtforms, wtforms-json
Solution
The easiest way is to wrap the `FormField` in a `FieldList`, with `max_entries` set to 1. `FieldList` also supports validators, but since `min_entries` is 0 by default you should not need any. The only annoyance will be that you will have to unwrap the inner form's data if it is available.
Problem
I'm using WTForms-JSON and processing nested forms. I'd like to make an inner form optional, but if the inner form is present, I'd like its fields to be required. The problem I'm running into is that `FormField` doesn't accept validators. (Although I'm using WTForms-JSON, I believe this applies to vanilla WTForms as well.) This code works but doesn't behave the way I want: ``` class InnerForm(Form): foo_id = IntegerField("Foo ID", [Required()]) class OuterForm(Form): inner = FormField(InnerForm) ``` The problem with the above code is that `inner` is implicitly required. Oddly, while `validate()` returns `False` when `inner` is omitted, `errors` is empty. This code doesn't doesn't work: ``` class InnerForm(Form): foo_id = IntegerField("Foo ID", [Required()]) class OuterForm(Form): inner = FormField(InnerForm, "Inner", [Optional()]) ``` The latter produces this error: ``` TypeError: FormField does not accept any validators. Instead, define them on the enclosed form. ``` My question is: how can I make `inner` optional, but require `foo_id` if `inner` is present?