#[serde_as(as = Option<...>)] does not allow field to be missing

[chyvonomys]

I'm trying to deserialize one optional field from comma-separated string:

#[serde_as]
#[derive(Deserialize, Debug)]
struct Query {
    search: Option<String>,
    #[serde_as(as = "Option<StringWithSeparator::<CommaSeparator, u32>>")]
    priority: Option<Vec<u32>>,
}

#[test]
fn test_queries() {
    let query1 = "priority=2,3";
    let q1: Result<Query, _> = serde_urlencoded::from_str(query1);
    println!("ok: {:?}", q1);
    let query2 = "search=ttt";
    let q2: Result<Query, _> = serde_urlencoded::from_str(query2);
    println!("fail: {:?}", q2);
}

But it doesn't allow that field to be missing:

ok: Ok(Query { search: None, priority: Some([2, 3]) })
fail: Err(Error { err: "missing field `priority`" })

[jonasbb]

This unfortunately a side effect of using deserialize_with. I see how I can better document the pitfall.

If you provide serde's default attribute, then it behaves as expected. It will deserialize a None in this case:

#[serde_as]
#[derive(Deserialize, Debug)]
struct Query {
    search: Option<String>,
    #[serde_as(as = "Option<StringWithSeparator::<CommaSeparator, u32>>")]
    #[serde(default)]
    priority: Option<Vec<u32>>,
}

If you do not care about the distinction between priority was provided but the list is empty and priority is missing, you can also remove the Option from priority:

#[serde_as(as = "StringWithSeparator::<CommaSeparator, u32>")]
#[serde(default, skip_serializing_if = "Vec::is_empty")]
priority: Vec<u32>,

The skip_serializing_if is only necessary if you also want to use Serialize.

I hope this helps you :)

[chyvonomys]

Thanks! First solution works for me.
It would be nice if that was the default behavior (without extra attribute), if possible.

[jonasbb]

Thanks for the feedback, I will consider that as an option.