在 Dynamo 中更新 object 的某些属性

Question

I have a DynamoDB table Policies and the object inside the table is as pages . The schema of pages looks like this -

{
    "pages": {
      "default": [
       {
        "label": "Blocked",
        "bodyMessage": "This content is restricted by your organisation.",
        "pageId": "defaultblocked",
        "pageTitle": "Content Blocked",
        "url": "some.s3.url"
       },
       {
        "label": "Warning",
        "bodyMessage": "This content is not suitable.",
        "pageId": "defaultwarning",
        "pageTitle": "Not Recommended",
        "url": "some.s3.url"
       },
       {
        "label": "Security Risk",
        "bodyMessage": "This content is risky.",
        "pageId": "defaultsecurityrisk",
        "pageTitle": "Risky Content",
        "url": "some.s3.url"
       },
       {
        "label": "Cloud Application",
        "bodyMessage": "This content is risky.",
        "pageId": "defaultcloudapplication",
        "pageTitle": "Cloud Application",
        "url": "some.s3.url"
       }
      ]
     }
}

I want to update pageTitle & bodyMessage of each object of pages.default but the values of each object would differ from each other.

For example, object with pageId as defaultblocked would have values as -

{
  "defaultblocked":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  }
}

So based on pageID the object from the pages.default should be selected & the attributes pageTitle & bodyMessage should be updated.

This is how the entire object would look like -

{
  "defaultblocked":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  },
    "defaultsecurityrisk":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  },
    "defaultwarning":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  },
    "defaultsecurityrisk":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  },
    "defaultcloudapplication":{
    "pageTitle":"Some title",
    "bodyMessage":"Some message"
  }
}

Answer 1

Updating a specific (nested) attribute in an item can be done using the update_item -method, specifically using an UpdateExpression like this:

SET parent.child =:val
Multiple update attributes can be concatenated in the same fashion.

An example on how you could set the title/message on two pages looks like this:

table.update_item(
        Key={"..": ".."},
        ExpressionAttributeNames={
            "#p": "pages",
            "#blocked": "defaultblocked",
            "#sec": "defaultsecurityrisk",
            "#title": "pageTitle",},
        ExpressionAttributeValues={
            ":val1": "title of blocked page",
            ":val2": "message on blocked page",
            ":val3": "title of security page",
            ":val4": "message on security page",
        },
        UpdateExpression="SET #p.#blocked.#title = :val1, #p.#blocked.#msg = :val2, #p.#sec.#title = :val3, #p.#sec.#msg = :val4",
        ReturnValues="UPDATED_NEW",
    )

Mind the ExpressionAttributeNames and ExpressionAttributeValues . The names always have to start with a # , and the values always start with a : . This is a DynamoDB convention to simplify the UpdateExpression, make it easier to read and make it less error prone.

Extending this to include all pages should be trivial, just add the pages to the UpdateExpression and include the values in the ExpressionAttributeNames/Values.

---

Edit :
One caveat of this approach, is that you can only update items of a map that already exist. For your case, if the existing item does not have a specific page yet, it cannot be updated like this.

To add a new page inside the object, you have to specify the entire object as a value:

table.update_item(
    Key={...},
    ExpressionAttributeNames={
        "#p": "pages",
        "#blocked": "defaultblocked",
        "#sec": "defaultsecurityrisk",
        "#title": "pageTitle",
        "#msg": "bodyMessage",
        "#new": "newpage"
    },
    ExpressionAttributeValues={
        ":val1": "title of blocked page",
        ":val2": "message on blocked page",
        ":val3": "title of security page",
        ":val4": "message on security page",
        ":new": {"pageTitle": "new title", "bodyMessage": "page of new body"}
    },
    UpdateExpression="SET #p.#new = :new, #p.#blocked.#title = :val1, #p.#blocked.#msg = :val2, #p.#sec.#title = :val3, #p.#sec.#msg = :val4",
    ReturnValues="UPDATED_NEW",
)