Since GSI auto updated from the base table, how do I make sure that the GSI partition and sort key values have prefix? For instance Email field is not part of primary key but I need email# prefix on partition key of GSI.

This is how I put item first time.

func (d DynamoDB) CreateUser(ctx context.Context, user store.User) error {
    model.PrimaryKey = store.PrimaryKey{
        PK: "user#" + user.ID,
        SK: "user#" + user.ID,
    }

    item, err := attributevalue.MarshalMap(user)
    if err != nil {
        return fmt.Errorf("marshal map: %w", err)
    }

    _, err = d.Client.PutItem(ctx, &dynamodb.PutItemInput{
        TableName: aws.String(d.Table),
        Item:      item,
    })
    if err != nil {
        return fmt.Errorf("put item: %w", err)
    }

    return nil
}

{
    "TableName": "BlogTable",
    "AttributeDefinitions": [
        {
            "AttributeName": "PK",
            "AttributeType": "S"
        },
        {
            "AttributeName": "SK",
            "AttributeType": "S"
        },
        {
            "AttributeName": "Email",
            "AttributeType": "S"
        }
    ],
    "KeySchema": [
        {
            "AttributeName": "PK",
            "KeyType": "HASH"
        },
        {
            "AttributeName": "SK",
            "KeyType": "RANGE"
        }
    ],
    "GlobalSecondaryIndexes": [
        {
            "IndexName": "BlogGlobalIndex1",
            "KeySchema": [
                {
                    "AttributeName": "Email",
                    "KeyType": "HASH"
                },
                {
                    "AttributeName": "SK",
                    "KeyType": "RANGE"
                }
            ],
            "Projection": {
                "ProjectionType": "ALL"
            },
            "ProvisionedThroughput": {
                "ReadCapacityUnits": 5,
                "WriteCapacityUnits": 5
            }
        }
    ],
    "ProvisionedThroughput": {
        "ReadCapacityUnits": 5,
        "WriteCapacityUnits": 5
    }
}

aws --profile localstack --endpoint-url http://localhost:4566 dynamodb scan --table-name BlogTable

{
    "Items": [
        {
            "SK": {
                "S": "user#1722201431023993"
            },
            "PK": {
                "S": "user#1722201431023993"
            },
            "ID": {
                "S": "1722201431023993"
            },
            "Email": {
                "S": "[email protected]"
            },
            "Name": {
                "S": "Roberto Medhurst"
            },
            "Password": {
                "S": "dentv9kkkjT7PDD"
            }
        }
    ],
    "Count": 1,
    "ScannedCount": 1,
    "ConsumedCapacity": null
}

aws --profile localstack --endpoint-url http://localhost:4566 dynamodb query 
    --table-name BlogTable 
    --index-name BlogGlobalIndex1 
    --key-condition-expression "Email = :email" 
    --expression-attribute-values  '{":email":{"S":"[email protected]"}}'

{
    "Items": [
        {
            "SK": {
                "S": "user#1722201431023993"
            },
            "PK": {
                "S": "user#1722201431023993"
            },
            "ID": {
                "S": "1722201431023993"
            },
            "Email": {
                "S": "[email protected]"
            },
            "Name": {
                "S": "Roberto Medhurst"
            },
            "Password": {
                "S": "dentv9kkkjT7PDD"
            }
        }
    ],
    "Count": 1,
    "ScannedCount": 1,
    "ConsumedCapacity": null
}