DynamoDB - Convenient way to read attribute of an Item

[sosnowski]

Hi!

I'm writing a pet project in Rust, using Dynamodb. My overall experience of using the SDK is very good, however, one thing seems to be quite verbose and problematic. It's reading the attribute value from the Item retrieved from the table. Maybe I'm missing something as my knowledge of Rust is relatively fresh.

Every time I read an attribute, I have to run two checks:
Check if the attribute actually exists

if let Some(name_attr) = db_item.get("name") {

}

And later, even if it actually exists, I have to check if it has the "expected" type:

if let Ok(name_value) = name_attr.as_s() {

}

Only then I can use the attribute value. The thing is, it's very verbose and I have to do it every time I want to read some attribute.
Additionally, I cannot use ? operator, as as_s() returns error that does not implement std::Error trait

the trait bound `AttributeValue: StdError` is not satisfied
required because of the requirements on the impl of `StdError` for `&AttributeValue`
required because of the requirements on the impl of `From<&AttributeValue>` for `anyhow::Error`

Is there a better way to do it? Some smart shortcut or method that I'm missing?

Thanks!
Damian

[rcoh]

And later, even if it actually exists, I have to check if it has the "expected" type:
This is a general problem with untyped data—you need to check that it exists and has the type you expect. There are crates like https://crates.io/crates/serde_dynamo/3.0.3/dependencies which provide an ergonomic wrapper around AttributeValue

The as_ methods return the untyped value so you can try to resolve a different type. You could do something like:

db_item
        .get("some_column_name")
        .ok_or("missing field")?
        .as_s()
        .map_err(|value| format!("expected a string, found: {:?}", value))?;

you could write a macro or helper function to shorten this pattern, but in general, Rust will force you to account for the case where typed data is not what you expect.

[sosnowski]

Actually serde_dynamo solves my problem quite nicely. Much better code ergonomics :) Thanks!

[github-actions[bot]]

Hello! Reopening this discussion to make it searchable.