Serialize a Uuid field in a struct to NULL if it is default()

[ronnybremer]

Scanning through the available options of serde_with and serde_as I found a way to deserialize a NULL value from JSON into a default() value for a Uuid field. However, for the other way around it looks like I would need to implement a customer serializer. Is there any easier way of doing that?

This is my struct:

#[derive(Debug, Serialize, PartialEq, Clone)]
struct Test {
   pub guid: Uuid,
}

I would like to get this output, when guid has been initialized as Uuid::default():

{
   "guid": null
}

#[serde(skip_serializing_if = "Uuid::is_nil")] does work, however, it eliminates the guid field altogether from the Json output, which is expected, but not what I need.

Thank you for your input.

[jonasbb]

There is nothing in the crate like this. There are no guarantees around Default::default() that can be made, so there are no pre-made conversions implemented.

I think something like this should work. I haven't compiled it. It basically converts the Uuid to Option<Uuid> based on the is_nil.

serde_with::serde_conv!(
    UuidNilAsNone,
    Uuid,
    |uuid: &Uuid| if uuid.is_nil() {None} else Some(uuid),
    |value: Option<Uuid>| -> Result<_, std::convert::Infallible> {
        Ok(value.unwrap_or_default())
    }
);

[ronnybremer]

Thank you @jonasbb for the clarification. So if I assume correctly, a comparison like

if val == T::default() {
}

could lead to unpredictable outcomes. It would be an expensive code anyway and AFAI there is no val.is_default() function.

I'll try your suggested code and gonna give you feedback.

[ronnybremer]

works perfectly, I had to clone() the Uuid in order to make it compile but I'm gonna optimize that. Thank you again @jonasbb.

[jonasbb]

Thats great to hear that this works for you. Default::default() can have different valid implementations for different types. One common value is something like empty/zero, i.e., None, empty Vec, 0. Generating a random UUID would be valid tool. All just to say, due to this unpredictability there aren't any premade converters that check for x == T::default().