How can I do key-agnostic deserialization of JSON objects? - json

Question

I am working with a less than ideal API that doesn't follow any rigid standard for sending data. Each payload comes with some payload info before the JSON, followed by the actual data inside which can be a single string or several fields.
As it stands right now, if I were to map every different payload to a struct I would end up with roughly 50 structs. I feel like this is not ideal, because a ton of these structs overlap in all but key. For instance, there are I believe 6 different versions of payloads that could be mapped to something like the following, but they all have different keys.
I have these two JSON examples:
{"key": "string"}
{"key2": "string"}
And I want to serialize both into this struct:
#[derive(Debug, Deserialize)]
struct SimpleString {
key: String,
}
The same can be said for two strings, and even a couple cases for three. The payloads are frustratingly unique in small ways, so my current solution is to just define the structs locally inside the function that deserializes them and then pass that data off wherever it needs to go (in my case a cache and an event handler)
Is there a better way to represent this that doesn't have so much duplication? I've tried looking for things like key-agnostic deserializing but I haven't found anything yet.

Answer 1

You can implement Deserialize for your type to decode a "map" and ignore the key name:
extern crate serde;
extern crate serde_json;
use std::fmt;
use serde::de::{Deserialize, Deserializer, Error, MapAccess, Visitor};
fn main() {
let a = r#"{"key": "string"}"#;
let b = r#"{"key2": "string"}"#;
let a: SimpleString = serde_json::from_str(a).unwrap();
let b: SimpleString = serde_json::from_str(b).unwrap();
assert_eq!(a, b);
}
#[derive(Debug, PartialEq)]
struct SimpleString {
key: String,
}
struct SimpleStringVisitor;
impl<'de> Visitor<'de> for SimpleStringVisitor {
type Value = SimpleString;
fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str("an object with a single string value of any key name")
}
fn visit_map<M>(self, mut access: M) -> Result<Self::Value, M::Error>
where
M: MapAccess<'de>,
{
if let Some((_, key)) = access.next_entry::<String, _>()? {
if access.next_entry::<String, String>()?.is_some() {
Err(M::Error::custom("too many values"))
} else {
Ok(SimpleString { key })
}
} else {
Err(M::Error::custom("not enough values"))
}
}
}
impl<'de> Deserialize<'de> for SimpleString {
fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
where
D: Deserializer<'de>,
{
deserializer.deserialize_map(SimpleStringVisitor)
}
}