Can't serialize/deserialize struct which contains Option<chrono::DateTime<Utc>> field.

[WindSoilder]

Take the following struct as example:

use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
use uuid::Uuid;

#[derive(Debug, Serialize, Deserialize)]
pub struct Authorization {
    #[serde(with = "bson::serde_helpers::uuid_as_binary")]
    client_id: Uuid,
    dbs: Vec<String>,
    colls: Option<Vec<String>>,
    #[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
    start_date: Option<DateTime<Utc>>,
    #[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
    end_date: Option<DateTime<Utc>>,
    name: String,
}

When I try to complile, I get the following error message:

mismatched types
= note: expected reference `&chrono::DateTime<Utc>`
              found reference `&'__a std::option::Option<chrono::DateTime<Utc>>`

I think this should work for start_date and end_date field just like colls field in the given Authorization struct, sorry for that I go through documentation and can't find out an answer, am I miss something?

[patrickfreed]

Unfortunately, using serde(with = "...") to compose with Option is not currently supported by serde (see serde-rs/serde#723). As a workaround, you can manually implement this via a custom (de)serialize module:

#[derive(Deserialize, Serialize, Debug)]
struct Dd {
    #[serde(with = "opt_chrono_datetime_as_bson_datetime")]
    #[serde(default)]
    date: Option<chrono::DateTime<Utc>>,
}

mod opt_chrono_datetime_as_bson_datetime {
    use chrono::Utc;
    use serde::{Deserialize, Deserializer, Serialize, Serializer};
    use mongodb::bson;

    #[derive(Serialize, Deserialize)]
    struct Helper(
        #[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
        chrono::DateTime<Utc>,
    );

    pub fn serialize<S>(
        value: &Option<chrono::DateTime<Utc>>,
        serializer: S,
    ) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        value.map(Helper).serialize(serializer)
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<Option<chrono::DateTime<Utc>>, D::Error>
    where
        D: Deserializer<'de>,
    {
        let helper: Option<Helper> = Option::deserialize(deserializer)?;
        Ok(helper.map(|Helper(external)| external))
    }
}

Now there does exist the serde_with crate which would make this much more ergonomic, but we currently don't have support for it. I filed RUST-1024 to track the work for introducing that integration. Once that ticket is done, the following should be possible:

#[serde_as]
#[derive(Debug, Serialize, Deserialize)]
struct MyData {
    #[serde_as(as = "Option<bson::DateTime>")]
    date: Option<chrono::DateTime<Utc>>
}

[xxated]

What if you try to import DateTime from MongoDB instead of chrono?
use mongodb::{
bson::{DateTime},
};

[patrickfreed]

Yep, using a bson::DateTime works just fine for this use case as well. The only consideration with that is you'd have to convert it to a chrono::DateTime any time you needed to use the chrono functionality.

BTW, the serde_with integration was contained in the 2.1.0-beta release, so the second example in my previous response should work now. Note that you'll need to enable the serde_with feature flag to use it. Going to close this issue out now that a workaround exists.

[ericpko]

@patrickfreed
I'm actually still getting an error:
error[E0277]: the trait bound bson::DateTime: SerializeAs<_> is not satisfied
error[E0277]: the trait bound bson::DateTime: DeserializeAs<'_, _> is not satisfied

even though I've tried your second option:

#[serde_as]
#[derive(Debug, Serialize, Deserialize)]
struct MyData {
    #[serde_as(as = "Option<bson::DateTime>")]
    date: Option<chrono::DateTime<Utc>>
}

Maybe I'm doing something obviously wrong that I'm not seeing?

[jonasbb]

@ericpko You need to use the same major version of serde_with as bson, currently this is v1. Be aware that serde_as got smarter in v2 regarding Option types. You will need to apply #[serde(default)] on these Option fields.

[ericpko]

@jonasbb This worked! Thank you!