当地图尚不存在时，如何设置 DynamoDB 地图属性值 [英] How to set a DynamoDB Map property value, when the map doesn't exist yet

问题描述

如何将属性更新"到 DynamoDB 行.例如.SET address.state = "MA" 对于某些项目，当 address 尚不存在时?

How do you "upsert" a property to a DynamoDB row. E.g. SET address.state = "MA" for some item, when address does not yet exist?

我觉得我遇到了先有鸡还是先有蛋的问题，因为 DynamoDB 不允许您提前定义草率的架构.

I feel like I'm having a chicken-and-egg problem because DynamoDB doesn't let you define a sloppy schema in advance.

如果 address DID 已经存在于该项目上，类型为 M(用于地图)，互联网告诉我我可以发出 UpdateExpression，如:

If address DID already exist on that item, of type M (for Map), the internet tells me I could issue an UpdateExpression like:

SET #address.#state = :value

#address、#state 和 :value 适当地映射到 address、state 和 MA.

with #address, #state, and :value appropriately mapped to address, state, and MA, respectively.

但如果 address 属性确实不 已经存在，则会报错:

But if the address property does not already exist, this gives an error:

'''ValidationException:更新表达式中提供的文档路径对更新无效'''

''' ValidationException: The document path provided in the update expression is invalid for update '''

所以.. 看来我要么需要:

So.. it appears I either need to:

1. 想办法更新"address.state(例如，SET address = {}; SET address.state = 'MA' 在单个命令中)

1. Figure out a way to "upsert" address.state (e.g., SET address = {}; SET address.state = 'MA' in a single command)

或

1. 发出三个 (!!!) 我尝试的往返，SET address = {}; 失败，然后再试一次.

1. Issue three (!!!) roundtrips in which I try it, SET address = {}; on failure, and then try it again.

如果是后者....如何设置空白地图?！?

If the latter.... how do I set a blank map?!?

呃..我喜欢 Dynamo，但除非我遗漏了一些明显的东西，否则这有点疯狂..

Ugh.. I like Dynamo, but unless I'm missing something obvious this is a bit crazy..

推荐答案

你可以通过两次往返来完成，第一次有条件地为 address 设置一个空映射，如果它不存在，第二个设置 state:

You can do it with two round trips, the first conditionally sets an empty map for address if it doesn't already exist, and the second sets the state:

db.update({
    UpdateExpression: 'SET #a = :value',
    ConditionExpression: 'attribute_not_exists(#a)',
    ExpressionAttributeValues: {
        ":value": {},
    },
    ExpressionAttributeNames: {
        '#a': 'address'
    }
}, ...);

然后:

db.update({
    UpdateExpression: 'SET #a.#b = :v',
    ExpressionAttributeNames: {
        '#a': 'address',
        '#b': 'state'
    },
    ExpressionAttributeValues: {
        ':v': 'whatever'
    }
}, ...);

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