DynamoDB Scan with FilterExpression in nodejs

Question

I'm trying to retrieve all items from a DynamoDB table that match a FilterExpression, and although all of the items are scanned and half do match, the expected items aren't returned.

I have the following in an AWS Lambda function running on Node.js 6.10:

var AWS = require("aws-sdk"),
    documentClient = new AWS.DynamoDB.DocumentClient();
function fetchQuotes(category) {
    let params = {
        "TableName": "quotient-quotes",
        "FilterExpression": "category = :cat",
        "ExpressionAttributeValues": {":cat": {"S": category}}
    };
    console.log(`params=${JSON.stringify(params)}`);
    documentClient.scan(params, function(err, data) {
        if (err) {
            console.error(JSON.stringify(err));
        } else {
            console.log(JSON.stringify(data));
        }
    });
}

There are 10 items in the table, one of which is:

{
  "category": "ChuckNorris",
  "quote": "Chuck Norris does not sleep. He waits.",
  "uuid": "844a0af7-71e9-41b0-9ca7-d090bb71fdb8"
}

When testing with category "ChuckNorris", the log shows:

params={"TableName":"quotient-quotes","FilterExpression":"category = :cat","ExpressionAttributeValues":{":cat":{"S":"ChuckNorris"}}}
{"Items":[],"Count":0,"ScannedCount":10}

The scan call returns all 10 items when I only specify TableName:

params={"TableName":"quotient-quotes"}
{"Items":[<snip>,{"category":"ChuckNorris","uuid":"844a0af7-71e9-41b0-9ca7-d090bb71fdb8","CamelCase":"thevalue","quote":"Chuck Norris does not sleep. He waits."},<snip>],"Count":10,"ScannedCount":10}

Answer 1

You do not need to specify the type ("S") in your ExpressionAttributeValues because you are using the DynamoDB DocumentClient. Per the documentation:

The document client simplifies working with items in Amazon DynamoDB by abstracting away the notion of attribute values. This abstraction annotates native JavaScript types supplied as input parameters, as well as converts annotated response data to native JavaScript types.

It's only when you're using the raw DynamoDB object via new AWS.DynamoDB() that you need to specify the attribute types (i.e., the simple objects keyed on "S", "N", and so on).

With DocumentClient, you should be able to use params like this:

const params = {
    TableName: 'quotient-quotes',
    FilterExpression: '#cat = :cat',
    ExpressionAttributeNames: {
        '#cat': 'category',
    },
    ExpressionAttributeValues: {
        ':cat': category,
    },
};

Note that I also moved the field name into an ExpressionAttributeNames value just for consistency and safety. It's a good practice because certain field names may break your requests if you do not.

Answer 2

I was looking for a solution that combined KeyConditionExpression with FilterExpression and eventually I worked this out. Where aws is the uuid. Id is an assigned unique number preceded with the text 'form' so I can tell I have form data, optinSite is so I can find enquiries from a particular site. Other data is stored, this is all I need to get the packet.

Maybe this can be of help to you:

let optinSite   = 'https://theDomainIWantedTFilterFor.com/';
let aws = 'eu-west-4:EXAMPLE-aaa1-4bd8-9ean-1768882l1f90';
let item = {
    TableName: 'Table',
    KeyConditionExpression: "aws = :Aw and begins_with(Id, :form)",
    FilterExpression: "optinSite = :Os",
    ExpressionAttributeValues: {   
        ":Aw" : { S: aws },
        ":form" : { S: 'form' },
        ":Os" : { S: optinSite }
    }
};