RUST-1540 Add Ability to deserialize String to ObjectId and i64 to DateTime

[borngraced]

For some reasons I see Bson doesn't deserialize Objected and DateTime to String and i64 respectively, so I think it'd be very helpful if there's an inbuilt helper fn to use with serde's deserialize_with macro attribute to automatically deserialize ObjectId to String and DateTime to i64..

Example

fn des_objectid<'de, D>(deserializer: D) -> Result<String, D::Error>
where
    D: de::Deserializer<'de>,
{
    let s: ObjectId = de::Deserialize::deserialize(deserializer)?;

    Ok(s.to_string())
}

then we'll be able to do something like this

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct User {
    #[serde(deserialize_with = "des_objectid")]
    pub _id: String,
    }

This is very useful for returning ObjectId as String and DateTime as i64

{ "id": "3895893bf983nc9..." }

instead of something like this

{ "id": { "$oid" : "3895893bf983nc9..."} }

[isabelatkinson]

Hi @borngraced, thanks for filling this issue! We have a serde_helpers module that contains functions you can use with the serialize_with and deserialize_with attributes. Included is a hex_string_as_object_id that allows conversion between ObjectIds and strings. We don't, however, have helpers for converting between DateTimes and i64s, so I've filed RUST-1540 to add them. If you're interesting in contributing to the BSON library, feel free to make a pull request implementing this functionality.