Golang convert interface{} to struct

Question

I want to improve the getCustomerFromDTO method in the code below, I need to create a struct from an interface{} and currently i need to marshall that interface to byte[] and then unmarshal the array to my struct - there must be a better way.

My use case is that I send structs via rabbitmq and to send them I use this general DTO wrapper that has additional domain specific data about them. When I receive the DTO from rabbit mq one layer below the message is unmarshaled to my DTO and then i need to get my struct from that DTO.

type Customer struct {
    Name string `json:"name"`
}

type UniversalDTO struct {
    Data interface{} `json:"data"`
    // more fields with important meta-data about the message...
}

func main() {
    // create a customer, add it to DTO object and marshal it
    customer := Customer{Name: "Ben"}
    dtoToSend := UniversalDTO{customer}
    byteData, _ := json.Marshal(dtoToSend)

    // unmarshal it (usually after receiving bytes from somewhere)
    receivedDTO := UniversalDTO{}
    json.Unmarshal(byteData, &receivedDTO)

    //Attempt to unmarshall our customer
    receivedCustomer := getCustomerFromDTO(receivedDTO.Data)
    fmt.Println(receivedCustomer)
}

func getCustomerFromDTO(data interface{}) Customer {
    customer := Customer{}
    bodyBytes, _ := json.Marshal(data)
    json.Unmarshal(bodyBytes, &customer)
    return customer
}

mkopriva · Accepted Answer · 2017-04-10 14:32:04Z

Before unmarshaling the DTO, set the Data field to the type you expect.

type Customer struct {
    Name string `json:"name"`
}

type UniversalDTO struct {
    Data interface{} `json:"data"`
    // more fields with important meta-data about the message...
}

func main() {
    // create a customer, add it to DTO object and marshal it
    customer := Customer{Name: "Ben"}
    dtoToSend := UniversalDTO{customer}
    byteData, _ := json.Marshal(dtoToSend)

    // unmarshal it (usually after receiving bytes from somewhere)
    receivedCustomer := &Customer{}
    receivedDTO := UniversalDTO{Data: receivedCustomer}
    json.Unmarshal(byteData, &receivedDTO)

    //done
    fmt.Println(receivedCustomer)
}

If you don't have the ability to initialize the Data field on the DTO before it's unmarshaled, you can use type assertion after the unmarshaling. Package encoding/json unamrshals interface{} type values into a map[string]interface{}, so your code would look something like this:

type Customer struct {
    Name string `json:"name"`
}

type UniversalDTO struct {
    Data interface{} `json:"data"`
    // more fields with important meta-data about the message...
}

func main() {
    // create a customer, add it to DTO object and marshal it
    customer := Customer{Name: "Ben"}
    dtoToSend := UniversalDTO{customer}
    byteData, _ := json.Marshal(dtoToSend)

    // unmarshal it (usually after receiving bytes from somewhere)
    receivedDTO := UniversalDTO{}
    json.Unmarshal(byteData, &receivedDTO)

    //Attempt to unmarshall our customer
    receivedCustomer := getCustomerFromDTO(receivedDTO.Data)
    fmt.Println(receivedCustomer)
}

func getCustomerFromDTO(data interface{}) Customer {
    m := data.(map[string]interface{})
    customer := Customer{}
    if name, ok := m["name"].(string); ok {
        customer.Name = name
    }
    return customer
}

The DTO unmarshaling is done by a package I use and it's the same for all received messages. In my application I receive an already unmarshalled DTO
And you don't have the ability to modify this package? is it 3rd party?
I can modify it, but since it's a general purpose package at the time i unmarshal the DTO i don't know the type of struct it carries in it's data field
See updated answer. Btw to keep it general purpose you can keep the Data field type as interface{} and all you need to do is to pre-initialize the value to a type that you expect back. This does not make it less general. You can pass the initialized value to a func in your package if you don't want to pre-initialize the DTO as well.. For example mypackage.UnmarshalDTO(byteData []byte, data interface{}) ... then in that func set data to the Data field and after unmarshal is done your data var will unmarshaled correctly.
@Tomas here's an example where the mypackageGetDTOFromSomewhere func does not need to know the type of the Data field, only the caller of that func needs to know the type.

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