How to add an additional data point to an instance of a struct when writing it to a file with Serde? [duplicate]

Question

A Person has a first_name and a last_name. How do I add a time of export to a CSV when writing an instance of Person to a CSV?

#[macro_use] extern crate serde_derive; extern crate csv; extern crate serde; use std::time::{SystemTime, UNIX_EPOCH}; #[derive(Debug, Serialize, Deserialize)] struct Person { first_name: String, last_name: String, } fn current_ms() -> u64 { let since_the_epoch = &SystemTime::now().duration_since(UNIX_EPOCH).unwrap(); let ms = since_the_epoch.as_secs() * 1000 + since_the_epoch.subsec_nanos() as u64 / 1_000_000; ms } fn main() { let person = Person { first_name: "Craig".to_string(), last_name: "David".to_string(), }; let file_path = std::path::Path::new("test.csv"); let mut wtr = csv::Writer::from_path(file_path).unwrap(); wtr.serialize(&person).unwrap(); println!("{}", current_ms()); }

The above code results in:

>>> cat test.csv first_name,last_name Craig,David

My objective is to generate a file which is in the format:

>>> cat test.csv time,first_name,last_name 1527661792118,Craig,David

Serde attributes do not have an option to add an additional field. In the above example the Person struct does not contain a time value. The time value is only required when the data is written to a CSV.

I believe your question is answered by the answers of How to transform fields during serialization using Serde?. If you disagree, please edit your question to explain the differences. Otherwise, we can mark this question as already answered. — Shepmaster
– Shepmaster, Commented May 30, 2018 at 10:06
@Shepmaster I've updated the post to explain why serde attributes do not appear to be a solution. — Greg
– Greg, Commented May 30, 2018 at 10:17
Did you read both parts of the answer? I know it's a very long answer, but read the part with "to create a separate struct". — Shepmaster
– Shepmaster, Commented May 30, 2018 at 10:37
I've reorganized that answer to make it clear that there are two solutions presented. — Shepmaster
– Shepmaster, Commented May 30, 2018 at 10:55
@Shepmaster Thanks! It took a while for me to understand how your solution worked however I got it in the end :) — Greg
– Greg, Commented May 30, 2018 at 11:14

Peter Hall · Accepted Answer · 2018-05-30 11:09:40Z

If you don't want to add a timestamp column to Person, you can create a separate struct for that and then join them together in a tuple:

#[derive(Debug, Deserialize, Serialize)] struct Timestamp { time: u64, } impl Timestamp { fn now() -> Timestamp { Timestamp { time: current_ms() } } } fn main() { let person = Person { first_name: "Craig".to_string(), last_name: "David".to_string(), }; let person_and_time = (Timestamp::now(), person); let file_path = std::path::Path::new("test.csv"); let mut wtr = csv::Writer::from_path(file_path).unwrap(); wtr.serialize(&person_and_time).unwrap(); }

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