Lambda Golang PutItem和MarshalMap到DynamoDB

Question

I'm trying to load data from Lambda using Golang into DynamoDB, however the marshalling method is just generating empty items. I have a type struct defined as below...

type Flight struct {
    id          string
    destination string
    airline     string
    time        string
    latest      string
}

I am then populating a slice as below.....

func PutToDynamo(Flights []Flight, kind string) {

Flights := []Flight{}

for _, row := range rows {
    columns := row.FindAll("td")

    f := columns[0].Text() //all these are strings
    a := columns[1].Text()
    i := columns[2].Text()
    t := columns[4].Text()
    l := columns[5].Text()

    flight := Flight{
        id:          i,
        destination: f,
        airline:     a,
        time:        t,
        latest:      l,
    }
    Flights = append(Flights, flight)

Which I then try and load into DynamoDB.

func PutToDynamo(flights []Flight, kind string) {
    for _, flight := range flights {
        av, err := dynamodbattribute.MarshalMap(flight)

        input := &dynamodb.PutItemInput{
            Item:      av,
            TableName: aws.String("flights"),
        }

        _, err = svc.PutItem(input)
        fmt.Println("FLIGHT: ", input.GoString())

If I print flight, I can see all the information I expect. However, input.GoString() is just returning the below...

 {
   Item: {
   },
   TableName: "flights"
 }

and I get an error thrown from DynamoDB in lambda.

ValidationException: One or more parameter values were invalid: Missing the key id in the item

Any ideas?? I've tried putting json:"id" etc in the struct, but no difference.

Thanks!

Answer 1

The fields from your structure are unexported, and this may be the reason why dynamo marshal is not marshaling it (quick look at the source code suggests that dynamo and json marshal methods are a bit similar). According to regular, golang json Marshal, from documentation:

The json package only accesses the exported fields of struct types (those that begin with an uppercase letter). Therefore only the the exported fields of a struct will be present in the JSON output.

There are two ways you can solve this:

a) Change your struct to have exported fields.

type Flight struct {
    Id          string
    Destination string
    Airline     string
    Time        string
    Latest      string
}

b) If you don't want to change your struct (for example if its referenced a lot in your code and refactoring would be a hassle), you can add a custom Marshal and Unmarhsal functions and hide your struct between a custom made export struct (code below is untested - its just to give a brief idea):

type Flight struct {
    id          string
    destination string
    airline     string
    time        string
    latest      string
}

type FlightExport struct {
        Id          string
        Destination string
        Airline     string
        Time        string
        Latest      string
}

func(f *Flight) MarshalJSON()([] byte, error) {
    fe: = &FlightExport {
        ID: f.id,
        Destination: f.destination,
        Airline: f.airline,
        Time: f.time,
        Latest: f.latest
    }

        return json.Marshal(fe)
}

func(f *Flight) UnmarshalJSON(data[] byte) error {
    var fe FlightExport
    err: = json.Unmarshal(data, &fe)
    if err != nil {
        return err
    }

    f.id = fe.ID
    f.destination = fe.Destination
    f.airline = fe.Airline
    f.time = fe.Time
    f.latest = fe.Latest

    return nil
}