InvalidMapKeyType when encoding a HashMap to bson

Question

I want to encode a HashMap<u64, usize> using bson::to_bson() in order to store it in MongoDB.

When I ran the code, it panicked and told me that InvalidMapKeyType(FloatingPoint(....)). Can't I use this method to encode HashMap in such a type?

Answer 1

The BSON library disallows all keys that are not strings. The BSON spec states that a document is a sequence of elements, each element must be preceded by a name, and a name can only be a string.

Changing your HashMap to use a string as the key should solve the problem.

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Your question doesn't make any sense to me. You state that you have a HashMap<u64, usize> but your error snippet states that it's because of a FloatingPoint!

This is why you should always create an MCVE and then provide it when asking a question. I created this sample which does exactly as you stated and I get a different error:

extern crate bson; // 0.8.0

use std::collections::HashMap;

fn main() {
    let mut thing = HashMap::new();
    thing.insert(0_u64, 1_usize);

    match bson::to_bson(&thing) {
        Ok(e) => println!("{:?}", e),
        Err(e) => println!("Got an error: {:?}, {}", e, e),
    }
}

Got an error: UnsupportedUnsignedType, BSON does not support unsigned type

If I change the HashMap to signed numbers, then I get the same class of error:

thing.insert(0_i64, 1_isize);

Got an error: InvalidMapKeyType(I64(0)), Invalid map key type: I64(0)

You can't even make a HashMap using a f64 as the key in Rust because it doesn't implement Hash or Eq, so I have no idea how you got that specific error.