Field

Builds an instance from the three members that always differ.

A checkbox. Every value a browser sends for a checked box means true; the only thing that means false is the key being absent, which is Field.absent's job rather than this one's.

A model's key.

Id[T] is core's and its Column is db's; this is the same UUID read through the same text, so it delegates rather than parsing a second time. A key is never rendered into a form (see Form), but it is still read through its own Field, which is what keeps one decoder for it rather than two.

An optional field. Empty text and an absent key are the same thing to a browser, so both are None here, and neither can ever be "is required".