json: cannot unmarshal object into Go value of type string" aws dynamodb

Question

I am following this post, so far the simple version is working but the section using AWS dynamodb is not working for me.

I have created an AWS dynamo table and inserted data

aws dynamodb create-table --table-name Services --attribute-definitions AttributeName=Name,AttributeType=S --key-schema AttributeName=Name,KeyType=HASH --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

aws dynamodb put-item --table-name Services --item '{"Name": {"S": "Lets Keep Safe"}, "ID": {"N": "3"}, "CreatedAt":  {"S": "2020-11-21"}}'

main.go

package main

import (
    "github.com/aws/aws-lambda-go/lambda"
)

// Service Names
type Service struct {
    Name      string `json:"name"`
    ID        int    `json:"id"`
    CreatedAt string `json:"created_at"`
}

func show(ID string) (*Service, error) {
    // Fetch a specific book record from the DynamoDB database. We'll
    // make this more dynamic in the next section.
    // ser, err := getItem(ID)
    ser, err := getItem("2")
    if err != nil {
        return nil, err
    }

    return ser, nil
}

func main() {
    lambda.Start(show)
}

db.go

package main

import (
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/dynamodb"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
)

// Declare a new DynamoDB instance. Note that this is safe for concurrent
// use.
var db = dynamodb.New(session.New(), aws.NewConfig().WithRegion("eu-west-1"))

func getItem(ID string) (*Service, error) {
    // Prepare the input for the query.
    input := &dynamodb.GetItemInput{
        TableName: aws.String("services"),
        Key: map[string]*dynamodb.AttributeValue{
            "ID": {
                N: aws.String(ID),
            },
        },
    }

    // Retrieve the item from DynamoDB. If no matching item is found
    // return nil.
    result, err := db.GetItem(input)
    if err != nil {
        return nil, err
    }
    if result.Item == nil {
        return nil, nil
    }

    // The result.Item object returned has the underlying type
    // map[string]*AttributeValue. We can use the UnmarshalMap helper
    // to parse this straight into the fields of a struct. Note:
    // UnmarshalListOfMaps also exists if you are working with multiple
    // items.
    ser := new(Service)
    err = dynamodbattribute.UnmarshalMap(result.Item, ser)
    if err != nil {
        return nil, err
    }

    return ser, nil
}

Invoking the lambda results in

{"errorMessage":"json: cannot unmarshal object into Go value of type string","errorType":"UnmarshalTypeError"}

Conor · Accepted Answer · 2020-11-26 22:57:16Z

working example based on the AWS dynamodb docs, the issue was it was attempting to unmarshal a null value, logging the output to cloudwatch caught this.

main.go

package main

import (
    "github.com/aws/aws-lambda-go/lambda"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbiface"
)

var (
    dynaClient dynamodbiface.DynamoDBAPI
)

func show() (*[]Service, error) {

    ser, err := getAllServices("Services")
    if err != nil {
        return nil, err
    }

    return ser, nil
}

func main() {
    lambda.Start(show)
}

db.go

package main

import (
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/dynamodb"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
    "github.com/aws/aws-sdk-go/service/dynamodb/expression"

    "fmt"
    "os"
)

// Service Names
type Service struct {
    Name      string `json:"name"`
    ID        int    `json:"id"`
    CreatedAt string `json:"created_at"`
}

var (
    sess = session.Must(session.NewSessionWithOptions(session.Options{
        SharedConfigState: session.SharedConfigEnable,
    }))

    // Create DynamoDB client
    svc = dynamodb.New(sess)
)

func getAllServices(tableName string) (*[]Service, error) {

    // Get back the title, year, and rating
    proj := expression.NamesList(expression.Name("Name"), expression.Name("ID"), expression.Name("CreatedAt"))

    expr, err := expression.NewBuilder().WithProjection(proj).Build()
    if err != nil {
        fmt.Println("Got error building expression:")
        fmt.Println(err.Error())
        os.Exit(1)
    }
    // snippet-end:[dynamodb.go.scan_items.expr]

    // snippet-start:[dynamodb.go.scan_items.call]
    // Build the query input parameters
    params := &dynamodb.ScanInput{
        ExpressionAttributeNames: expr.Names(),
        ProjectionExpression:     expr.Projection(),
        TableName:                aws.String(tableName),
    }

    // Make the DynamoDB Query API call
    result, err := svc.Scan(params)
    if err != nil {
        fmt.Println("Query API call failed:")
        fmt.Println((err.Error()))
        os.Exit(1)
    }

    item := new([]Service)
    err = dynamodbattribute.UnmarshalListOfMaps(result.Items, item)
    return item, nil

}

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