How to deserialize in Rust (using serde) optional json parameter that can be either string or string array

Question

I am new to Rust and I am trying to deserialize JSON data using serde library. I have following JSON structure:

{
    “foo”: “bar”,
    “speech”: “something”
}

or

{
    “foo”: “bar”,
    “speech”: [“something”, “something else”]
}

or

{
    “foo”: “bar”,
}

I.e. speech is optional and it can be either string or array of strings.

I can handle deserializing string/array of string using following approach:

#[derive(Debug, Serialize, Deserialize)]
   struct foo {
   pub foo: String,
   #[serde(deserialize_with = "deserialize_message_speech")]
   speech: Vec<String>
}

I can also handle deserializing optional string/string array attribute using approach:

#[derive(Debug, Serialize, Deserialize)]
struct foo {
   pub foo: String,
   #[serde(skip_serializing_if = "Option::is_none")]
   speech: Option<Vec<String>>
}

or

struct foo {
   pub foo: String,
   #[serde(skip_serializing_if = "Option::is_none")]
   speech: Option<String>
}

But combining it all together simply does not work. It seems deserialize_with does not work properly with Option type. Can somebody advice most straightforward and trivial way how to implement this (serde can be pretty complex, I have seen some crazy stuff :) )?

Answer 1

Try using an Enum type for the speech field:

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum Speech {
    Str(String),
    StrArray(Vec<String>),   
}

#[derive(Debug, Serialize, Deserialize)]
   struct foo {
   pub foo: String,
   speech: Option<Speech>,
}

Enum is the go-to way to represent a variant type in Rust. See https://serde.rs/enum-representations.html for more detail.

Answer 2

with #[serde(untagged)] it works!

use serde_json;
use std::result::Result;
use std::error::Error;
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
#[serde(untagged)]
enum Speech {
    Str(String),
    StrArray(Vec<String>),   
}

#[derive(Debug, Serialize, Deserialize)]
struct Foo {
   pub foo: String,
   #[serde(skip_serializing_if = "Option::is_none")]
   speech: Option<Speech>,
} 

fn main() -> Result<(), Box<dyn Error>> {

    let json1 = r#"
    {
        "foo": "bar",
        "speech": "something"
    }
    "#;    

    let json2 = r#"
    {
        "foo": "bar",
        "speech": ["something", "something else"]
    }
    "#;    


    let json3 = r#"
    {
        "foo": "bar"
    }
    "#;    

    let foo1: Foo = serde_json::from_str(json1)?;
    let back_to_str_foo1 = serde_json::to_string(&foo1).unwrap();
    println!("foo1 {:#?}", foo1);
    println!("back_to_str_foo1 {}", back_to_str_foo1);

    let foo2: Foo = serde_json::from_str(json2)?;
    let back_to_str_foo2 = serde_json::to_string(&foo2).unwrap();
    println!("foo1 {:#?}", foo2);
    println!("back_to_str_foo2 {}", back_to_str_foo2);


    let foo3: Foo = serde_json::from_str(json3)?;
    let back_to_str_foo3 = serde_json::to_string(&foo3).unwrap();
    println!("foo1 {:#?}", foo3);
    println!("back_to_str_foo3 {}", back_to_str_foo3);

    Ok(())
}