dynamodel

This library provides a derive macro to implement conversions between your object and HashMap<String, AttributeValue>.

Derive macro Dynamodel

The Dynamodel derive macro implements these three traits to use aws-sdk-dynamodb more comfortably.

• Into<HashMap<String, AttributeValue>>
• TryFrom<HashMap<String, AttributeValue>>
• The AttributeValueConvertible trait enables the types that implement it to be converted from and to AttributeValue.

#[derive(Dynamodel)]        Convertible
struct YourStruct { ... }  <===========>  HashMap<String, AttributeValue>

#[derive(Dynamodel)]    Convertible
enum YourEnum { ... }  <===========>  HashMap<String, AttributeValue>

Requirements to use Dynamodel

To use the Dynamodel macro, all types of your object's fields must implement the AttributeValueConvertible trait.

By default, these types automatically implement the AttributeValueConvertible trait, so no additional code is required when using these types.

The last row of the above table shows that once you apply the Dynamodel macro to your object, it also implements the AttributeValueConvertible trait for your object.

So, you can create nested structures of objects that apply the Dynamodel macro.

If you want to use additional types, you need to implement the AttributeValueConvertible trait for your type.

Usage

use dynamodel::Dynamodel;
use std::collections::HashMap;
use aws_sdk_dynamodb::types::AttributeValue;

#[derive(Dynamodel, Debug, Clone, PartialEq)]
struct Person {
    first_name: String,
    last_name: String,
    age: u8,
}

let person = Person {
    first_name: "Kanji".into(),
    last_name: "Tanaka".into(),
    age: 23,
};

let item: HashMap<String, AttributeValue> = [
    ("first_name".to_string(), AttributeValue::S("Kanji".into())),
    ("last_name".to_string(), AttributeValue::S("Tanaka".into())),
    ("age".to_string(), AttributeValue::N("23".into()))
].into();

// Convert from Person into HashMap<String, AttributeValue>.
let converted: HashMap<String, AttributeValue> = person.clone().into();
assert_eq!(converted, item);

// Convert from HashMap<String, AttributeValue> into Person.
// This conversion uses std::convert::TryFrom trait, so this returns a Result.
let converted: Person = item.try_into().unwrap();
assert_eq!(converted, person);

Modifying the default behavior

Like the Serde crate, you can modify the default behavior through attributes like this.

use dynamodel::{Dynamodel, ConvertError};
use std::collections::HashMap;
use aws_sdk_dynamodb::{types::AttributeValue, primitives::Blob};

// Vec<u8> is converted to AttributeValue::L by default,
// but this case, the `data` field is converted to AttributeValue::B.
#[derive(Dynamodel)]
struct BinaryData {
    #[dynamodel(into = "to_blob", try_from = "from_blob")]
    data: Vec<u8>
}

fn to_blob(value: Vec<u8>) -> AttributeValue {
    AttributeValue::B(Blob::new(value))
}

fn from_blob(value: &AttributeValue) -> Result<Vec<u8>, ConvertError> {
    value.as_b()
        .map(|b| b.clone().into_inner())
        .map_err(|err| ConvertError::AttributeValueUnmatched("B".to_string(), err.clone()))
}

The function definition must meet these conditions.

Example

Single-table design

The following diagram shows that both Video and VideoStats are stored in the same table.

#[derive(Dynamodel)]
#[dynamodel(extra = "VideoStats::sort_key", rename_all = "PascalCase")]
struct VideoStats {
    #[dynamodel(rename = "PK")]
    id: String,
    view_count: u64,
}

impl VideoStats {
    fn sort_key(&self) -> HashMap<String, AttributeValue> {
        [
            ("SK".to_string(), AttributeValue::S("VideoStats".into())),
        ].into()
    }
}

And suppose you want to add a VideoComment object that is sortable by timestamp, like this.

#[derive(Dynamodel)]
#[dynamodel(rename_all = "PascalCase")]
struct VideoComment {
    #[dynamodel(rename = "PK")]
    id: String,
    #[dynamodel(rename = "SK", into = "sort_key", try_from = "get_timestamp")]
    timestamp: String,
    content: String,
}

fn sort_key(timestamp: String) -> AttributeValue {
    AttributeValue::S(format!("VideoComment#{timestamp}"))
}

fn get_timestamp(value: &AttributeValue) -> Result<String, ConvertError> {
    value.as_s()
        .map(|v| v.split('#').last().unwrap().to_string())
        .map_err(|e| ConvertError::AttributeValueUnmatched("S".into(), e.clone()))
}

More features

For more features, refer to this wiki.

License

This software is released under the MIT License.