How to Marshall any data type to string and Unmarshal from string to specific data type on condition

Question

type State struct {
    Type      string    `json:"type" validate:"required"`
    Value     string    `json:"value"`
}

I have a struct like this. I need to pass the state to the API in different types.

eg: state can be either { type: 'boolean', value: true } or { type: 'string', value: 'ABC' }

And I'm storing it ( the value ) as a string in db.

Then when I'm passing from an API, I need to set the particular value considering the type (not as a string).

Same as this { type: 'boolean', value: true } , { type: 'string', value: 'ABC' }

How can I achieve this with Marshalling and Unmarshalling?

Answer 1

You can define marshal and unmarshal logic in your code.

type State struct {
    Type  string      `json:"type" validate:"required"`
    Value interface{} `json:"value"`
}

func (s *State) UnmarshalJSON(b []byte) (err error) {
    tmpMap := map[string]string{}
    err = json.Unmarshal(b, &tmpMap)
    if err != nil {
        return err
    }
    if tmpMap["type"] == "" {
        return errors.New("type not present")
    }
    if tmpMap["value"] == "" {
        return errors.New("value not present")
    }
    if tmpMap["type"] == "string" {
        s.Type = "string"
        s.Value = tmpMap["value"]
    } else if tmpMap["type"] == "boolean" {
        s.Type = "boolean"
        s.Value = tmpMap["value"] == "true"
    } else {
        //TODO implements other type
        return errors.New(fmt.Sprintf("Unknown type %s", tmpMap["type"]))
    }
    return nil
}

See https://play.golang.org/p/BnD7HAuURzJ

Similarly, you can define MarshalJSON method on State struct to handle data serialization.

func (c State) MarshalJSON() ([]byte, error)

To store State db in single column in db you can implement

func (c State) Value() (driver.Value, error)

To build a state from DB columns you need to implement Scan method as

func (e *State) Scan(value interface{}) error