是否可以在 DynamoDB 中进行条件放置或更新? [英] Is it possible to do a conditional put or update in DynamoDB?

问题描述

假设我在 DynamoDB 中存储具有以下结构的记录:

Let's say that I am storing records with following structure in DynamoDB:

{    
    "id": "57cf5b43-f9ec-4796-9de6-6a50f556cfd8",
    "created_at": "2015-09-18T13:27:00+12:00",
    "count": 3
}

现在，是否有可能在一个请求中实现以下目标:

Now, is it possible to achieve the following in one request:

• 如果具有给定 id 的记录不存在，则应使用 count = 1 创建它
• 如果该 id 的记录存在，则计数器正在更新.

• if the record with given id doesn't exist it should be created with count = 1
• if the record for that id exists the counter is being updated.

目前我正在查询记录是否存在，并根据结果执行 put 或 update.将它折叠成一个操作会很好.

Currently I'm doing a query to check if the record exist and depending on the result I do a put or an update. It would be nice to fold that into a single operation.

推荐答案

您可以使用 UpdateItem API 和 UpdateExpression 因为您的用例.由于此处的 count 将是 Number 类型，因此您可以使用 SET 或 ADD 表达式:

You can do this with UpdateItem API and the UpdateExpression because of your use case. Since count will be a Number type here, you can use the SET or ADD expressions:

ADD 的文档告诉您可以将它用于 Number 类型(强调我的):

The documentation for ADD tells you that you can use it for Number types (emphasis mine):

• ADD - 如果该属性尚不存在，则将指定的值添加到项目.如果属性确实存在，那么 ADD 的行为取决于属性的数据类型:

• ADD - Adds the specified value to the item, if the attribute does not already exist. If the attribute does exist, then the behavior of ADD depends on the data type of the attribute:

• 如果现有属性是一个数字，并且如果 Value 也是一个数字，那么 Value 会以数学方式添加到现有属性中.如果 Value 为负数，则从现有属性中减去.

• If the existing attribute is a number, and if Value is also a number, then Value is mathematically added to the existing attribute. If Value is a negative number, then it is subtracted from the existing attribute.

如果您使用 ADD 来增加或减少更新前不存在的项目的数值，DynamoDB 使用 0 作为初始值.类似地，如果您将 ADD 用于现有的item 增加或减少更新前不存在的属性值，DynamoDB 使用 0 作为初始值.例如，假设您要更新的项目没有名为 itemcount 的属性，但您决定将数字 3 添加到此属性中.DynamoDB 将创建 itemcount 属性，将其初始值设置为 0，最后添加 3.结果将是项目中的新 itemcount 属性，值为 3.

If you use ADD to increment or decrement a number value for an item that doesn't exist before the update, DynamoDB uses 0 as the initial value. Similarly, if you use ADD for an existing item to increment or decrement an attribute value that doesn't exist before the update, DynamoDB uses 0 as the initial value. For example, suppose that the item you want to update doesn't have an attribute named itemcount, but you decide to ADD the number 3 to this attribute anyway. DynamoDB will create the itemcount attribute, set its initial value to 0, and finally add 3 to it. The result will be a new itemcount attribute in the item, with a value of 3.

对于您的示例，您可以让您的 UpdateExpression 为 ADD #c :n，其中 :n 有一个 ExpressionAttributeValue/code> 类型，1 是值，#c 具有 ExpressionAttributeName 替换 count.您需要为 count 使用占位符，因为它是 保留字.

For your example, you could have your UpdateExpression be ADD #c :n, where :n has an ExpressionAttributeValue of the Number type, 1 is the value, and #c has the ExpressionAttributeName substitution for count. You need to use a placeholder for count because it is a reserved word.

查看使用更新表达式修改项目和属性

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