Rust custom deserialize implementation

Question

I'm having a hard time understanding how to implement deserialize for a custom mapping using Rust's serde. I'd be glad if someone could help me with this example:

I've got the following struct:

#[derive(Debug, Clone, PartialEq)]
pub struct ConnectorTopics {
    pub name: String,
    pub topics: Vec<String>,
}

And the JSON data comes in the following format:

{
  "test-name": {
    "topics": [
      "topic1",
      "topic2"
    ]
  }
}

As you can see, name field is a wrapper for the topics, so in my case this should deserialize to:

let _ = ConnectorTopics {
    name: "test-name".into(),
    topics: vec!["topic1".into(), "topic2".into()]
}

My first attempt was to use a custom structure inside Deserialize implementation, however, that wouldn't compile and doesn't seem to be the correct approach.

impl<'de> Deserialize<'de> for ConnectorTopics {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        #[derive(Debug, Deserialize)]
        struct Inner {
            topics: Vec<String>,
        }

        let a = deserializer.deserialize_map(HashMap<String, Inner>).unwrap();

        

        let value = Deserialize::deserialize::<HashMap<String, Inner>>(deserializer)?;

        let (connector, inner) = value.iter().nth(0).ok_or("invalid")?.0;

        Ok(ConnectorTopics {
            name: connector,
            topics: vec![],
        })
    }
}

Answer 1

What you was doing is the correct approach but your json is quite odd:

use serde::de;
use serde::Deserialize;
use std::fmt;

#[derive(Debug, Clone, PartialEq)]
pub struct ConnectorTopics {
    pub name: String,
    pub topics: Vec<String>,
}

#[derive(Debug, Deserialize)]
struct Inner {
    topics: Vec<String>,
}

impl<'de> de::Deserialize<'de> for ConnectorTopics {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: de::Deserializer<'de>,
    {
        struct ConnectorTopicsVisitor;

        impl<'de> de::Visitor<'de> for ConnectorTopicsVisitor {
            type Value = ConnectorTopics;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("ConnectorTopics")
            }

            fn visit_map<V>(self, mut map: V) -> Result<Self::Value, V::Error>
            where
                V: de::MapAccess<'de>,
            {
                if let Some(key) = map.next_key()? {
                    let value: Inner = map.next_value()?;
                    if let Some(_) = map.next_key::<&str>()? {
                        Err(de::Error::duplicate_field("name"))
                    } else {
                        Ok(Self::Value {
                            name: key,
                            topics: value.topics,
                        })
                    }
                } else {
                    Err(de::Error::missing_field("name"))
                }
            }
        }

        deserializer.deserialize_map(ConnectorTopicsVisitor {})
    }
}

fn main() {
    let input = r#"{
      "test-name": {
        "topics": [
          "topic1",
          "topic2"
        ]
      }
    }"#;

    let result: ConnectorTopics = serde_json::from_str(input).unwrap();

    let expected = ConnectorTopics {
        name: "test-name".into(),
        topics: vec!["topic1".into(), "topic2".into()],
    };

    assert_eq!(result, expected);
}

Answer 2

You can use a custom function for it:

pub fn deserialize_connector_topics(data: &str) -> Result<ConnectorTopics> {
    let value: Value = serde_json::from_str(data)?;
    if let Some(object) = value.as_object() {
        let mut it = object.into_iter();
        if let Some((name, topics)) = it.next() {
            let topics: Vec<String> = serde_json::from_value(topics.get("topics").unwrap().clone())?;
            return Ok(ConnectorTopics {
                name: name.to_string(),
                topics: topics,
            });
        }
    };
    Err(Error::custom("Invalid ConnectorTopics data"))
}

Please, notice it is pretty ad-hoc and it even has some unwraps and (maybe unnecessary) clones here and there. You should consider modifying it for your needs. But as an example, it should be enough.

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