Write once properties about fluent-kit HOT 2 OPEN

Transferring this to FluentKit since that's where the code would live for such a feature.

It should be possible to declare field defaults in Fluent 4, which could be an improvement to how we handle timestamps. Those types of properties could be made to only support reading. But, I'm not sure how write-once properties would work. If you need to be able to write the property ever (even if only once), then the property needs to be writable. There's not really a way to tell the compiler to only make it writable once.

To give some context, this is how setting a read only property w/ a default could work:

struct Planet: Model {
    let id = Field<Int>("id")
    let name = Field<String>("name")
    let createdAt = ReadOnlyField<Date>("createdAt", default: .currentDate)
}

let planet: Row<Planet> // from DB
try planet.get(\.createdAt) // Date
try planet.set(\.createdAt, to: ...) // Compiler error, ambiguous use of `set`

I'm not sure I love that API though since it's within the realm of possibility that you might need to update the createdAt field for whatever reason. It seems like it should be up to the developer whether to do that or not.

FWIW, ReadOnlyField could be declared entirely in user code without needing any internal Fluent support.

from fluent-kit.

In "regular" Swift, there is a way to declare write-once properties: let. My idea was similar; have a kind of property that doesn't expose a set method, with the only place to set its value in an initializer or when decoding from a database result set. However, I now see that set is (needs to, I guess) declared on the model itself, not on each individual field.

Explicit manual updates could still be done via a SQL UPDATE command, but by avoiding a set method we could avoid accidentally changing the corresponding property on the model and then calling save() with that changed property — again, similar to how let works.

from fluent-kit.