Unmarshall DynamoDB JSON

Question

Given some DynamoDB JSON via a DynamoDB NewImage stream event, how do I unmarshall it to regular JSON?

{"updated_at":{"N":"146548182"},"uuid":{"S":"foo"},"status":{"S":"new"}}

Normally I would use AWS.DynamoDB.DocumentClient, however I can't seem to find a generic Marshall/Unmarshall function.

Sidenote: Do I lose anything unmarshalling DynamoDB JSON to JSON and back again?

If others come looking for the python/boto version of this problem, here's the relevant question: stackoverflow.com/questions/36558646/… — villasv
– villasv, Commented Dec 7, 2017 at 15:10

giaour · Accepted Answer · 2017-06-15 21:34:19Z

65

You can use the AWS.DynamoDB.Converter.unmarshall function. Calling the following will return { updated_at: 146548182, uuid: 'foo', status: 'new' }:

AWS.DynamoDB.Converter.unmarshall({ "updated_at":{"N":"146548182"}, "uuid":{"S":"foo"}, "status":{"S":"new"} })

Everything that can be modeled in DynamoDB's marshalled JSON format can be safely translated to and from JS objects.

edited Jun 15, 2017 at 21:34

answered Jun 14, 2017 at 6:01

giaour

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7 Comments

hendry Over a year ago

ARGH.. what an awful API that it requires that M thing aws.DynamoDB.Converter.output({ 'M': record.dynamodb.NewImage })

giaour Over a year ago

AWS.DynamoDB.Converter.output is the part of the DocumentClient that translates DynamoDB AttributeValue objects to JSON-style objects. I assumed the M was part of the data you got in the stream event.

giaour Over a year ago

I opened a PR to improve the API by adding a marshall function that doesn't require the M key, and it was included in version 2.71.0 of the SDK.

alext Over a year ago

Worked fine for me as of April 2020 using the AWS SDK Javascript API. Thanks a lot!

Steve S. Over a year ago

I know this is an old answer but this is not working for me and it seems related to the fact that a query can return a mix of json that can be decorated or not with datatyping. For exemple this doesn't work : { "Items": [ { "entityType": { "S": "member" } } ], "Count": 1, "ScannedCount": 1 }... will result in : { Items: undefined, Count: undefined, ScannedCount: undefined } .. even array [] alone doesn't work.

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burntsugar · Accepted Answer · 2021-02-03 05:31:38Z

AWS SDK for JavaScript version 3 (V3) provides nice methods for marshalling and unmarshalling DynamoDB records reliably.

const { marshall, unmarshall } = require("@aws-sdk/util-dynamodb"); const dynamo_json = { "updated_at": { "N": "146548182" }, "uuid": { "S": "foo" }, "status": { "S": "new" } }; const to_regular_json = unmarshall(dynamo_json); const back_to_dynamo_json = marshall(to_regular_json);

Output:

// dynamo_json { updated_at: { N: '146548182' }, uuid: { S: 'foo' }, status: { S: 'new' } } // to_regular_json { updated_at: 146548182, uuid: 'foo', status: 'new' } // back_to_dynamo_json { updated_at: { N: '146548182' }, uuid: { S: 'foo' }, status: { S: 'new' } }

For this to work for me in TypeScript, I had to cast unmarshall(recordImage as { [key: string]: AttributeValue }); // AttributeValue = import { AttributeValue } from "@aws-sdk/client-dynamodb";
Wow! You have NO IDEA how hard this is to find.. I don't see this documented much if at all or I am blind in the DynamoDB docs and this seems like a must have tool when working with it!

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