Using `serde::Serialize` with `Option<chrono::DateTime>`

Question

When trying to serialize Option<chrono::DateTime<Utc>> I'm encountering an error:

error[E0308]: mismatched types
  --> src/main.rs:39:14
   |
39 |     #[derive(Serialize, Debug)]
   |              ^^^^^^^^^ expected struct `DateTime`, found enum `std::option::Option`
   |
   = note: expected reference `&DateTime<Utc>`
              found reference `&'__a std::option::Option<DateTime<Utc>>`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

Code (Playground):

use chrono::{serde::ts_seconds, DateTime, NaiveDate, Utc};
use serde::Serialize;

fn main() {
    let test_struct = TestStruct {
        a: 2.45,
        date: Some(DateTime::from_utc(
            NaiveDate::from_ymd(2000, 1, 1).and_hms(1, 1, 1),
            Utc,
        )),
    };
    let string = serde_json::to_string(&test_struct).unwrap();
    println!("default: {}", string);
    
    #[derive(Serialize, Debug)]
    struct TestStruct {
        pub a: f32,
        #[serde(with = "ts_seconds")]
        pub date: Option<DateTime<Utc>>,
    }
}

Looking at chrono::ts_seconds and serde_with I have no idea where to move forward here.

I would really appreciate any help with this.

Answer 1

Chrono already has a function for Option<DateTime<Utc>>, namely chrono::serde::ts_seconds_option.

#[derive(Serialize, Debug)]
struct TestStruct {
    pub a: f32,
    #[serde(with = "ts_seconds_option")]
    pub date: Option<DateTime<Utc>>,
}

The solution with serde_with looks like this:

#[serde_as]
#[derive(Serialize, Debug)]
struct TestStruct {
    pub a: f32,
    #[serde_as(as = "Option<DurationSeconds<i64>>")]
    pub date: Option<DateTime<Utc>>,
}

Answer 2

You can write your own wrapper and combine it with serialize_with and skip_serializing_if:

pub fn serialize_dt<S>(
    dt: &Option<DateTime<Utc>>, 
    serializer: S
) -> Result<S::Ok, S::Error> 
where
    S: Serializer {
    match dt {
        Some(dt) => ts_seconds::serialize(dt, serializer),
        _ => unreachable!(),
    }
}

#[derive(Serialize, Debug)]
struct TestStruct {
    pub a: f32,
    #[serde(serialize_with = "serialize_dt", skip_serializing_if  = "Option::is_none")]
    pub date: Option<DateTime<Utc>>,
}

Playground