Updating GSIs with strings like it's possible with numbers

index.js

const DynamoDB = require("aws-sdk/clients/dynamodb");
const client = new DynamoDB.DocumentClient({
    region: "us-east-1",
    endpoint: "http://localhost:8000"
});

/**
 * withNumbers demonstrates the update of a "composite" gsi sortkey without knowledge of the current
 * composite value at the time of update. This works runs without error and operates as expected.
 */
async function withNumbers() {
    const pk = 500;
    const sk = 200;
    const table = "with_numbers_example";

    // clear prior runs
    await client.delete({
        Key: { pk, sk },
        TableName: table
    }).promise();

    // create a new item
    await client.put({
        Item: {
            number1: 500,
            number2: 200,
            number3: 10,
            number4: 35,
            number5: 5,
            pk: 500,
            sk: 200,
            gsi1pk: 10, // single attribute "composite", should mirror `number3`
            gsi1sk: 40, // two attribute composite, should mirror `number4` + `number5`
        },
        TableName: table
    }).promise();

    // An update to `number5` should update `number5` (the attribute itself) and recalculate `gsi1sk`
    await client.update({
        UpdateExpression: "SET #gsi1sk = :number5 + #number4, #number5 = :number5 + #number4",
        ExpressionAttributeNames: {
            "#gsi1sk": "gsi1sk",
            "#number4": "number4",
            "#number5": "number5",
        },
        ExpressionAttributeValues: {
            ":number5": 10,
        },
        Key: { pk, sk },
        TableName: table
    }).promise();

    const results = await client.query({
        IndexName: "gsi1pk-gsi1sk-index",
        KeyConditionExpression: "#pk = :pk and #sk = :sk",
        ExpressionAttributeNames: {
            "#pk": "gsi1pk",
            "#sk": "gsi1sk"
        },
        ExpressionAttributeValues: {
            ":pk": 10, // still equal to original `number3` value
            ":sk": 45 // now equal original `number4` value (35) + new `number5` value (10)
        },
        TableName: table
    }).promise();

    console.log(JSON.stringify(results, null, 4));
}

/**
 * withStrings demonstrates the update of a "composite" gsi sortkey without knowledge of the current
 * composite value at the time of update. This uses the same approach as `withNumbers` but does not
 * have comparable functionality.
 */
async function withStrings() {
    const pk = "mdinescu";
    const sk = "#user";
    const table = "with_strings_example";

    // clear prior runs
    await client.delete({
        Key: { pk, sk },
        TableName: table
    }).promise();

    // create a new item
    await client.put({
        Item: {
            username: "mdinescu",
            team: "alexa_games",
            city: "irvine",
            state: "ca",
            pk: "mdinescu",
            sk: "#user",
            gsi1pk: "alexa_games", // single attribute "composite", should mirror `team`
            gsi1sk: "irvine#ca", // two attribute composite, should mirror `city` + `state`
        },
        TableName: table
    }).promise();

    // An update to `city` should update `city` (the attribute itself) and recalculate `gsi1sk`
    // Note: doesn't include "#" delimiter to keep things close to the original `withNumbers` example.
    await client.update({
        UpdateExpression: "SET #gsi1sk = :city + #state, #city = :city + #state",
        ExpressionAttributeNames: {
            "#gsi1sk": "gsi1sk",
            "#city": "city",
            "#state": "state",
        },
        ExpressionAttributeValues: {
            ":city": "oakland",
        },
        Key: { pk, sk },
        TableName: table
    }).promise();

    const results = await client.query({
        IndexName: "gsi1pk-gsi1sk-index",
        KeyConditionExpression: "#pk = :pk and #sk = :sk",
        ExpressionAttributeNames: {
            "#pk": "gsi1pk",
            "#sk": "gsi1sk"
        },
        ExpressionAttributeValues: {
            ":pk": "alexa_games", // still equal to original `team` value
            ":sk": "oakland#ca" // now equal new `city` value ("oakland") + original `state` value ("ca")
        },
        TableName: table
    }).promise();

    console.log(JSON.stringify(results, null, 4));
}

async function main() {
    await withNumbers();
    await withStrings();
}

main().catch(console.log);

with_numbers_example.json

{
  "KeySchema":[
    {
      "AttributeName":"pk",
      "KeyType":"HASH"
    },
    {
      "AttributeName":"sk",
      "KeyType":"RANGE"
    }
  ],
  "AttributeDefinitions":[
    {
      "AttributeName":"pk",
      "AttributeType":"N"
    },
    {
      "AttributeName":"sk",
      "AttributeType":"N"
    },
    {
      "AttributeName":"gsi1pk",
      "AttributeType":"N"
    },
    {
      "AttributeName":"gsi1sk",
      "AttributeType":"N"
    }
  ],
  "GlobalSecondaryIndexes":[
    {
      "IndexName":"gsi1pk-gsi1sk-index",
      "KeySchema":[
        {
          "AttributeName":"gsi1pk",
          "KeyType":"HASH"
        },
        {
          "AttributeName":"gsi1sk",
          "KeyType":"RANGE"
        }
      ],
      "Projection":{
        "ProjectionType":"ALL"
      }
    }
  ],
  "BillingMode":"PAY_PER_REQUEST",
  "TableName": "with_numbers_example"
}

with_strings_example.json

{
  "KeySchema":[
      {
          "AttributeName":"pk",
          "KeyType":"HASH"
      },
      {
          "AttributeName":"sk",
          "KeyType":"RANGE"
      }
  ],
  "AttributeDefinitions":[
      {
          "AttributeName":"pk",
          "AttributeType":"S"
      },
      {
          "AttributeName":"sk",
          "AttributeType":"S"
      },
      {
          "AttributeName":"gsi1pk",
          "AttributeType":"S"
      },
      {
          "AttributeName":"gsi1sk",
          "AttributeType":"S"
      }
  ],
  "GlobalSecondaryIndexes":[
      {
          "IndexName":"gsi1pk-gsi1sk-index",
          "KeySchema":[
              {
                  "AttributeName":"gsi1pk",
                  "KeyType":"HASH"
              },
              {
                  "AttributeName":"gsi1sk",
                  "KeyType":"RANGE"
              }
          ],
          "Projection":{
              "ProjectionType":"ALL"
          }
      }
  ],
  "BillingMode":"PAY_PER_REQUEST",
  "TableName": "with_strings_example"
}