Updating object fields given dynamic JSON

[iamhoodie]

I have a use case where I would like to update an objects specific fields given dynamic JSON.

For example:

struct Transmitter {
enabled: bool, // user modifiable (setpoint)
deadband: f64, // user modifiable (setpoint)
alarm: bool, // user modifiable (setpoint)
accum: i32, // non-modifiable by user

}

From a GUI, the user would send a "setpoint" or modification containing only what was changed to the instance. For example, if the user wanted to enable the alarm field, the GUI would send the following as JSON: "{'alarm': true}". Alternatively, if the user enabled the alarm and change the deadband, the following would be sent: "{'alarm': true, 'deadband': 500}".

Is it possible or is there a way to modifiable fields of a struct given only JSON. Possibly a use case in serde?

Thanks!

[alice]

You can make a struct like this:

#[derive(Deserialize)]
struct TransmitterUpdate {
    #[serde(default)]
    enabled: Option<bool>,

    #[serde(default)]
    deadband: Option<f64>,

    #[serde(default)]
    alarm: Option<bool>,
}

Then you can update each field that isn't None.

Notice that #[serde(default)] will insert the value of Default::default() when the field is missing, and for options, that happens to be None.

[krdln]

I've used a following pattern once:

• deserialize current state of struct to json
• deserialize "patch" to json
• combine two jsons
• deserialize from json

#[derive(Serialize, Deserialize)]
struct Config { /* ... */ }; // The struct to update – in your case, a Transmitter

use serde_json::Value;

fn amend(config: Config, new_rules: &Value) -> crate::Result<Config> {
    let config: Value = serde_json::to_value(&config).unwrap();

    let mut config: BTreeMap<String, Value> = serde_json::from_value(config).unwrap();
    let new_rules: BTreeMap<String, Value> = serde_json::from_value(new_rules.clone())?;
    for (k, v) in new_rules {
        config.insert(k, v);
    }

    let config: Value = serde_json::to_value(&config).unwrap();
    Ok(serde_json::from_value(config)?)
}

It's not the fastest way, and I certainly consider it more of a hack than a solution, but hey, it works!

Also, there was a recent discussion on reddit on similar topic https://www.reddit.com/r/rust/comments/fbe0h3/serde_deserializing_into_an_existing_object/

[iamhoodie]

That doesn't look too bad actually...It's not something that will be happening all that often anyways so I'm not too concerned with speed. I'll definitely play around with that method!

[iamhoodie]

I ended up using the pointer_mut method of serde_json. Worked as needed. Seemed much cleaner!

Thanks