Add bind attribute support

[DCjanus]

Example:

#[derive(Debug, Deserialize)]
pub struct Config {
    pub application: Application,
    // other fields
}

#[derive(Debug, Deserialize)]
pub struct Application {
    pub database: Database,
}

#[derive(Debug, Deserialize)]
pub struct Database {
    #[serfig(bind = "DATABASE_URL")]
    pub url: String,
}

When I was using sqlx, to work with sqlx-cli, command line to manage database migrations, I have to provide database connection options to it, which means I have to put database URL in command line args or environment variable, mostly I'd like to put this in file in name .env.

But in my application, the path to the field may not be database.url, I hope to bind the field in specific path.

[Xuanwo]

Nice feature request! I'm thinking about adding similar features too.

However, this feature seems not trivial.

serfig rely on serde to do deserialization. Inside serfig, we will:

• Deserialize all values into Self
• Serialize into serde_bridge::Value
• Merge all the values
• Deserialize the final value into Self

So there is small space for us to bind something. I have two ideas so far.

Wrappers for different source

In databend, to make our MetaConfig compatible, I'm adding wrappers to load values from different sources in refactor(meta/config): Adapt RFC Config Backward Compatibility

pub fn load() -> MetaResult<Self> {
    let arg_conf = Self::parse();

    let mut builder: serfig::Builder<Self> = serfig::Builder::default();

    // Load from the config file first.
    {
        let config_file = if !arg_conf.config_file.is_empty() {
            arg_conf.config_file.clone()
        } else if let Ok(path) = env::var("METASRV_CONFIG_FILE") {
            path
        } else {
            "".to_string()
        };

        builder = builder.collect(from_file(Toml, &config_file));
    }

    // Then, load from env.
    let cfg_via_env: ConfigViaEnv = serfig::Builder::default()
        .collect(from_env())
        .build()
        .map_err(|e| MetaError::InvalidConfig(e.to_string()))?;
    builder = builder.collect(from_self(cfg_via_env.into()));

    // Finally, load from args.
    builder = builder.collect(from_self(arg_conf));

    builder
        .build()
        .map_err(|e| MetaError::InvalidConfig(e.to_string()))
}

This way needs a lot of From/Into and is not suitable for a simple DATABASE_URL.

Add bind support in serde-env

Maybe we can bind it in serde-env instead?

It's similar to serde's alias but only affects serde-env and will not change other serde formats.

#[derive(Debug, Deserialize)]
pub struct Config {
    pub application: Application,
    // other fields
}

#[derive(Debug, Deserialize)]
pub struct Application {
    pub database: Database,
}

#[derive(Debug, Deserialize)]
pub struct Database {
    #[serde_env(bind = "database_url")]
    pub url: String,
}

This way seems better, but we still need some investigation on how to implement it.

---

What do you think?

[DCjanus]

It's hard to impl than I thought, I've no idea yet.