Serialize/Deserialize CSV with nested enum/struct with serde in Rust - csv

Question

I want to serialize/deserialize a CSV file with variable row length and content, like the following:
./test.csv
Message,20200202T102030,Some message content
Measurement,20200202T102031,10,30,40,2
AnotherMeasurement,20200202T102034,0,2
In my opinion, the easiest way to represent this is the following enum:
#[derive(Debug, Serialize, Deserialize)]
pub enum Record {
Message { timestamp: String, content: String }, // timestamp is String because of simplicity
Measurement { timestamp: String, a: u32, b: u32, c: u32, d: u32 },
AnotherMeasurement { timestamp: String, a: u32, b: u32 },
}
Cargo.toml
[dependencies]¬
csv = "^1.1.6"¬
serde = { version = "^1", features = ["derive"] }
Running the following
main.rs
fn example() -> Result<(), Box<dyn Error>> {
let mut rdr = csv::ReaderBuilder::new()
.has_headers(false)
.delimiter(b',')
.flexible(true)
.double_quote(false)
.from_path("./test.csv")
.unwrap();
for result in rdr.deserialize() {
let record: Record = result?;
println!("{:?}", record);
}
Ok(())
}
fn write_msg() -> Result<(), Box<dyn Error>> {
let msg = Record::Message {
timestamp: String::from("time"),
content: String::from("content"),
};
let mut wtr = csv::WriterBuilder::new()
.has_headers(false)
.flexible(true)
.double_quote(false)
.from_writer(std::io::stdout());
wtr.serialize(msg)?;
wtr.flush()?;
Ok(())
}
fn main() {
if let Err(err) = example() {
println!("error running example: {}", err);
}
if let Err(err) = write_msg() {
println!("error running example: {}", err);
}
}
prints
error running example: CSV deserialize error: record 0 (line: 1, byte: 0): invalid type: unit variant, expected struct variant
error running example: CSV write error: serializing enum struct variants is not supported
Is there an easy solution to do this with serde and csv? I feel like I missed one or two serde attributes, but I was not able to find the right one in the documentation yet.
EDITS
Netwave suggested adding the #[serde(tag = "type")] attribute. Serializing now works, Deserializing gives the following error:
error running example: CSV deserialize error: record 0 (line: 1, byte: 0): invalid type: string "Message", expected internally tagged enum Record
Research I did that did not lead to a solution yet
Is there a way to "flatten" enums for (de)serialization in Rust?
https://docs.rs/csv/1.1.6/csv/tutorial/index.html
Custom serde serialization for enum type
https://serde.rs/enum-representations.html

Answer 1

Make your enum tagged (internally tagged specifically):
#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "type")]
pub enum Record {
Message { timestamp: String, content: String }, // timestamp is String because of simplicity
Measurement { timestamp: String, a: u32, b: u32, c: u32, d: u32 },
AnotherMeasurement { timestamp: String, a: u32, b: u32 },
}
Playground