Do multiple DynamoDb queries using Lambda function

Question

I want to write some logic like fetch post with time limit 1 day(created date) and if the list of items are smaller  then 20 then fetch the post within 2 days(created date).

like:

```
const params = {
  TableName: 'Todo-xxxxxxxxxxxxxxx-dev',
  IndexName: 'title-createdAt-index',
  "Limit": 10,
  KeyConditionExpression: '#title =:value and #createdAt BETWEEN :start AND :end',
  ExpressionAttributeValues: { ':value': 'hi',
                                  ':start':'2023-06-29T03:47:35.343Z',
                                  ':end':  '2023-06-30T03:47:35.343Z'
  },
  ExpressionAttributeNames: { '#title': 'title',
                              '#createdAt':'createdAt'
                          }
 };
```
if the above code returns 10 list items then I will fetch  with time limit ':start':'2023-06-28T03:47:35.343Z', ':end':  '2023-06-29T03:47:35.343Z' and if it returns 6 items then I will fetch items again 1 days ago. Is this a good practice?

Answer

According to your question,

You have a DynamoDB Table which has a partition key **userId** and sort key **createdAt**. These attributes will be combined as Primary Key. I created an example table **posts** with the following items
![Enter image description here](/media/postImages/original/IMBy46JB2wThq2tZoCyFejqA)

You are saying that you want to fetch posts within 1 day and if they are less than 20, you need to fetch posts within 2 days. This is not a good practice.
I think what you want is to fetch 20 latest posts of a user.

This is the lambda code that fetches last 20 items(posts) of a user.

**Note:- runtime: nodejs 18x, Used AWS SDK for JavaScript v3**

```
import { DynamoDBClient, QueryCommand, ScanCommand } from "@aws-sdk/client-dynamodb";
export const ddbClient = new DynamoDBClient();

export const handler = async(event) => {
    const endDate = '2023-01-07T17:18:44.219Z';
    const params = {
        TableName: 'posts',
        KeyConditionExpression: 'userId = :userId AND createdAt <= :endDate',
        ExpressionAttributeValues: {
          ':endDate': {S: endDate},
          ':userId': {S: '1'}
        },
        Limit:20,
        ScanIndexForward: false,
    };
   
    const result = await ddbClient.send(new QueryCommand(params));
    
    console.log(result.Items);
        
    const response = {
    statusCode: 200,
    body: JSON.stringify(result.Items),
    };
    return response;
    
};

```

Here, I have hardcoded the userId and endDate. You can pass it through event object.
**Limit: 20** means it give only 20 Items and **ScanIndexForward: false** gives the items in descending order

Hope this answer helps

Do multiple DynamoDb queries using Lambda function

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