PATCH http request validation of single field

I have a struct snippet which is something like this

type Talent struct {
	Code               string   `json:"code" gorm:"type:varchar(10);not null"`
	FirstName          string   `json:"firstName" example:"Ravi" gorm:"type:varchar(50)"`
	LastName           string   `json:"lastName" example:"Sharma" gorm:"type:varchar(50)"`
	Email              string   `json:"email" example:"john@doe.com" gorm:"type:varchar(100)"`
	Contact            string   `json:"contact" example:"1234567890" gorm:"type:varchar(15)"`
	AcademicYear       *uint8    `json:"academicYear" example:"1" gorm:"type:tinyint(2)"`
	Type               *uint8   `json:"talentType" example:"4" gorm:"type:tinyint(2)"`
	Resume             *string  `json:"resume" gorm:"type:varchar(200)"`
	ExperienceInMonths *uint    `json:"experienceInMonths"`
	Image              *string  `json:"image" gorm:"type:varchar(200)"`
	Gender             *string  `json:"gender" gorm:"type:varchar(50)"`
	DateOfBirth        *string  `json:"dateOfBirth" gorm:"type:date"`
}

I want to update one field at a time, so I am sending only one field in json to the API as a PATCH http request and in backend(Golang) I am converting the json to struct where in only that field in json will have value in the struct... all other fields will be nil.

For example I am sending firstName, now when I convert it to struct only the firstName field is filled. Now I want to validate that field.. like firstName is a compulsary field so it will have two validations.. one for checking if its empty and one for the max number of characters.

Like wise all fields have some validations. Now how do I validate only that particular field. Having switch cases is what I had considered but is there any other optimal way to do this in golang??

i come across the same situation and solved it this way:

added a GetJsonFields method to get the json keys in a string array

type Book struct {
	Id           int    `json:"id"`
	Title        string `json:"title" validate:"required"`
	Descr        string `json:"description"`
	ThumbnailUrl string `json:"thumbnail_url" validate:"required"`
	CreatedAt    string `json:"created_at" validate:"required"`
}

func (b Book) GetJsonFields() []string
 {
	var jsonFields []string
	val := reflect.ValueOf(b)
	for i := 0; i < val.Type().NumField(); i++ {
		jsonFields = append(jsonFields, val.Type().Field(i).Tag.Get("json"))

	}

	return jsonFields
}

i also wrote a small helper function to check for the string to contain:

func contains(stringSlice []string, text string) bool {
	for _, a := range stringSlice {
		if a == text {
			return true
		}
	}
	return false
}

With that it is easy to write a simple validation logic

        // get url param id
		book_id := mux.Vars(r)["id"]

		// decode body
		var generic map[string]interface{}
		json.NewDecoder(r.Body).Decode(&generic)

		// get json fields
		jsonFields := new(models.Book).GetJsonFields()

		// loop through the body
		// if one key does not match -> return http.StatusBadRequest
		for k, _ := range generic {
			if contains(jsonFields, k) == false {
				rw.WriteHeader(http.StatusBadRequest)
				response := responses.BookResponse{Status: http.StatusBadRequest, Message: "error", Data: map[string]interface{}{"data": "a provided field is unknown."}}
				json.NewEncoder(rw).Encode(response)
				return
			}
		}