AWS SDK for JavaScriptを使ってDynamoDBテーブルの空文字列を取得する方法

はじめに

• 空文字列をscanする方法が分からなかった
• size()でできそうなのでやってみた

なんかググってすぐ出てこなかったのでやってみることにした。

やってみたこと

• 事前準備
  • SAMPLEというテーブルを作成（プライマリキー：id）
• 実施

ddb-scan.js

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

const DOC_CLIENT = new AWS.DynamoDB.DocumentClient({apiVersion: '2012-08-10'});
const TABLE_NAME = 'SAMPLE';

const main = async() =>{
    // データ投入
    const Items = [
        {"value": "DOP","id": "BBB","next": "DDD","prev": "CCC"},
        {"value": "SOA","id": "DDD","next": "EEE","prev": "DDD"},
        {"value": "SAP","id": "AAA","next": "CCC","prev": ""},
        {"value": "DVA","id": "EEE","next": "FFF","prev": "DDD"},
        {"value": "SAA","id": "CCC","next": "BBB","prev": "AAA"},
        {"value": "SCS","id": "FFF","next": "","prev": "EEE"}
    ]
    await Promise.all(
        Items.map(async (elm) => {
            const params = {
                TableName: TABLE_NAME,
                Item: elm
            };
            await DOC_CLIENT.put(params).promise();
        })
    );

    // データ参照
    const TABLE_PARAMS = {
        TableName: TABLE_NAME,
        ExpressionAttributeNames: {
            "#next": "next"
        },  
        ExpressionAttributeValues: {
            ":len": 0
        },  
        FilterExpression: "size (#next) = :len",
    };

    const scannedData = await DOC_CLIENT.scan(TABLE_PARAMS, () => {}).promise();
    console.log(JSON.stringify(scannedData));  
}

main();    

実行結果：

$ node ddb-scan.js 
{"Items":[{"value":"SCS","id":"FFF","next":"","prev":"EEE"}],"Count":1,"ScannedCount":6}

まとめ

• 空文字列はsizeで取得できる
• 文字数の条件は ExpressionAttributeValues を使う必要がある

参考

• Comparison Operator and Function Reference - Amazon DynamoDB https://docs.aws.amazon.com/ja_jp/amazondynamodb/latest/developerguide/Expressions.OperatorsAndFunctions.html