在我的 Lambda 函数中，我只想有条件地将项目放入我的 DynamoDB 中，前提是该值已经不存在。我看到了多个不同的来源，他们使用了这个 ConditionExpression，我不知道这有什么问题。
body = await dynamo.put({
TableName: 'polit-stream',
Item: {
urlPath: data.urlPath,
},
ConditionExpression: "attribute_not_exists(urlPath)"
}).promise();
即使我的二级索引值 (urlPath) 已经存在，put 也将始终成功。
完整代码：
const AWS = require('aws-sdk');
const crypto = require("crypto");
const dynamo = new AWS.DynamoDB.DocumentClient();
/**
* Demonstrates a simple HTTP endpoint using API Gateway. You have full
* access to the request and response payload, including headers and
* status code.
*
* To scan a DynamoDB table, make a GET request with the TableName as a
* query string parameter. To put, update, or delete an item, make a POST,
* PUT, or DELETE request respectively, passing in the payload to the
* DynamoDB API as a JSON body.
*/
exports.handler = async(event, context) => {
let body;
let statusCode = '200';
const headers = {
'Content-Type': 'application/json',
'Access-Control-Allow-Origin': '* '
}
const data = JSON.parse(event.body);
const generateUUID = () => crypto.randomBytes(16).toString("hex");
try {
switch (event.httpMethod) {
case 'DELETE':
body = await dynamo.delete(JSON.parse(event.body)).promise();
break;
case 'GET':
body = await dynamo.scan({
TableName: 'db',
IndexName: 'urlPath',
FilterExpression: "urlPath = :urlPath",
ExpressionAttributeValues: {
":urlPath": event.queryStringParameters.urlPath
}
},
function(data) {
}).promise();
break;
case 'POST':
body = await dynamo.put({
TableName: 'db',
Item: {
id: generateUUID(),
name: data.name,
date: data.date,
place: data.place,
goals: data.goals,
type: data.type,
org: data.org,
email: data.email,
urlPath: data.urlPath,
createdAt: new Date().toISOString()
},
ConditionExpression: "attribute_not_exists(urlPath)"
}).promise();
break;
case 'PUT':
body = await dynamo.update(JSON.parse(event.body)).promise();
break;
default:
throw new Error(`Unsupported method "${event.httpMethod}"`);
}
}
catch (err) {
statusCode = '400';
body = err.message;
}
finally {
body = JSON.stringify(body);
}
return {
statusCode,
body,
headers,
};
};