extern crate serde;
#[macro_use]
extern crate serde_derive;
extern crate serde_json;

use serde_json::Value;
use std::collections::HashMap;

#[derive(Serialize, Deserialize, Debug, Clone)]
pub struct MyStruct {
    custom: std::collections::HashMap<String, Value>,
}

fn main() {
    let instance: MyStruct = serde_json::from_str(
        r#"{"custom": { "a":12345, "b":"213", "c":true, "object_field": {} }}"#,
    )
    .unwrap();

    println!("{:?}", instance.custom.to_string_string_map());
}

trait ToStringStringMap {
    fn to_string_string_map(&self) -> HashMap<String, String>;
}

impl ToStringStringMap for HashMap<String, Value> {
    fn to_string_string_map(&self) -> HashMap<String, String> {
        self.iter()
            .map(|(k, v)| {
                let v = match v.clone() {
                    e @ Value::Number(_) | e @ Value::Bool(_) => e.to_string(),
                    Value::String(s) => s,
                    _ => {
                        println!(r#"Warning : Can not convert field : "{}'s value to String, It will be set to empty string."#, k);
                        "".to_string()
                    }
                };

                (k.clone(), v)
            })
            .collect()
    }
}