Unmarshal map[string]DynamoDBAttributeValue into a struct

Question

I'm trying to set-up an AWS-lambda using aws-sdk-go that is triggered whenever a new user is added to a certain dynamodb table.

Everything is working just fine but I can't find a way to unmarshal a map map[string]DynamoDBAttributeValue like:

{
    "name": {
        "S" : "John"
    },
    "residence_address": {
        "M": {
            "address": {
                "S": "some place"
            }
        }
    }
}

To a given struct, for instance, a User struct. Here is shown an example of unsmarhaling a map[string]*dynamodb.AttributeValue into a given interface, but I can't find a way to do the same thing with map[string]DynamoDBAttributeValue even though these types seem to fit the same purposes.

map[string]DynamoDBAttributeValue is returned by a events.DynamoDBEvents from package github.com/aws/aws-lambda-go/events. This is my code:

package handler

import (
    "context"
    "github.com/aws/aws-lambda-go/events"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
    "github.com/aws/aws-sdk-go/service/dynamodb"
)

func HandleDynamoDBRequest(ctx context.Context, e events.DynamoDBEvent) {

    for _, record := range e.Records {

        if record.EventName == "INSERT" {

            // User Struct
            var dynamoUser model.DynamoDBUser

            // Of course this can't be done for incompatible types
            _ := dynamodbattribute.UnmarshalMap(record.Change.NewImage, &dynamoUser)
        }

    }

}

Of course, I can marshal record.Change.NewImage to JSON and unmarshal it back to a given struct, but then, I would have to manually initialize dynamoUser attributes starting from the latter ones.

Or I could even write a function that parses map[string]DynamoDBAttributeValue to map[string]*dynamodb.AttributeValue like:

func getAttributeValueMapFromDynamoDBStreamRecord(e events.DynamoDBStreamRecord) map[string]*dynamodb.AttributeValue {
    image := e.NewImage
    m := make(map[string]*dynamodb.AttributeValue)
    for k, v := range image {
        if v.DataType() == events.DataTypeString {
            s := v.String()
            m[k] = &dynamodb.AttributeValue{
                S : &s,
            }
        }
        if v.DataType() == events.DataTypeBoolean {
            b := v.Boolean()
            m[k] = &dynamodb.AttributeValue{
                BOOL : &b,
            }
        }
        // . . .
        if v.DataType() == events.DataTypeMap {
            // ?
        }
    }
    return m
}

And then simply use dynamodbattribute.UnmarshalMap, but on events.DataTypeMap it would be quite a tricky process.

Is there a way through which I can unmarshal a DynamoDB record coming from a events.DynamoDBEvent into a struct with a similar method shown for map[string]*dynamodb.AttributeValue?

Answer 1

I tried the function you provided, and I met some problems with events.DataTypeList, so I managed to write the following function that does the trick:

// UnmarshalStreamImage converts events.DynamoDBAttributeValue to struct
func UnmarshalStreamImage(attribute map[string]events.DynamoDBAttributeValue, out interface{}) error {

    dbAttrMap := make(map[string]*dynamodb.AttributeValue)

    for k, v := range attribute {

        var dbAttr dynamodb.AttributeValue

        bytes, marshalErr := v.MarshalJSON(); if marshalErr != nil {
            return marshalErr
        }

        json.Unmarshal(bytes, &dbAttr)
        dbAttrMap[k] = &dbAttr
    }

    return dynamodbattribute.UnmarshalMap(dbAttrMap, out)

}