nodejs DynamoDB Scan by Mapped Items

Question

I have a DynamoDB table where I would like to return all Items whose Review.ID mapped value is equal to 123.

Item: {
  id: 1,
  review: {
    Id: 123,
    step1: 456,
    step2: 789,
    step3: 1234,
    },
  // Add more items here
},
Item: {
  id: 2,
  review: {
    Id: 123,
    step1: 999,
    step2: 888,
    step3: 777,
    },
  // Add more items here
},

Ideal Return Example:

id: 1,
review: {
 - id: 123,
 - step: 456,
 - step2: 789,
 - step3: 1234,
}
id: 2,
review: {
  - id: 123,
 - step: 999,
 - step2: 888,
 - step3: 777,
}

This is my current code in NodeJS

exports.handler = async (event, context, callback) => {

 const params = {
    FilterExpression: "review.Id = :review",
    ExpressionAttributeValues: {
        ":review": 123,
    },
  TableName: "sometable"
 };


   let promise = dynamoDb.scan(params).promise();
   let result = await promise;
   let data = result.Items;
    if (result.LastEvaluatedKey) {
        params.ExclusiveStartKey = result.LastEvaluatedKey;
        data = data.concat(await (params));
    }


    // create a response
      const response = {
      statusCode: 200,
      body: JSON.stringify(data),
    };
    callback(null, response);

};

Running this code I get an empty result [].

Any help on this would be greatly appreciated.

Answer 1

You haven't shown all your code but I am going to suggest that the problem is that your dynamoDb client is the low-level DynamoDB Client rather than the high-level Document Client . They use different ways of supplying attribute values. You're using the low-level DynamoDB client but indicating attribute values as if you're using the high-level Document Client.

Here's an example that contrasts the two client approaches:

const AWS = require('aws-sdk');
AWS.config.update({ region: 'us-east-1' });

// High-level Document Client
const dc = new AWS.DynamoDB.DocumentClient();

// High-level scan parameters for Document Client
const paramsDC = {
  FilterExpression: "review.Id = :review",
  ExpressionAttributeValues: {
    ":review": 123,
  },
  TableName: "sometable"
};

// Low-level DynamoDB Client
const db = new AWS.DynamoDB();

// Low-level scan parameters for DynamoDB Client
const paramsDB = {
  FilterExpression: "review.Id = :review",
  ExpressionAttributeValues: {
    ":review": { "N": "123" },
  },
  TableName: "sometable"
};

(async() => {
  // High-level scan
  const resultDC = await dc.scan(paramsDC).promise();
  console.log('DC Items:', JSON.stringify(resultDC.Items));

  // Low-level scan
  const resultDB = await db.scan(paramsDB).promise();
  console.log('DB Items:', JSON.stringify(resultDB.Items));
})();

This results in:

DC Items: [{"review":{"step":999,"id":123},"id":2},{"review":{"step":456,"id":123},"id":1}]
DB Items: [{"review":{"M":{"step":{"N":"999"},"id":{"N":"123"}}},"id":{"N":"2"}},{"review":{"M":{"step":{"N":"456"},"id":{"N":"123"}}},"id":{"N":"1"}}]

Note that the low-level DynamoDB Client response looks a little different but is actually just a different (unmarshalled) representation of the same results. It includes "id":{"N":"1"} which says that id is a number with value 1 where the Document Client shows "id":1 . The low-level results includes "M": {... } which are maps, while the high-level Document Client unmarshalls these to JavaScript objects for you automatically.

Bottom-line:

• the Document Client is newer
• the Document Client marshalls and unmarshalls data to/from JavaScript native objects
• use the Document Client