Custom Unmarshal with String Split in Go

When dealing with JSON data, the need often arises to transform or customize the unmarshaling process for specific data types. In this case, we want to split a JSON string containing comma-separated values into a []string slice during unmarshalling.

To achieve this, let's implement a custom unmarshaler for the []string type:

type strslice []string

func (ss *strslice) UnmarshalJSON(data []byte) error {
    var s string
    if err := json.Unmarshal(data, &s); err != nil {
        return err
    }
    *ss = strings.Split(s, "-")
    return nil
}

This custom unmarshaler takes the raw JSON data as input and converts it to a slice of strings by splitting it on the specified delimiter ("-" in this case).

In our original struct, we can now use this custom type for the Subjects field:

type Student struct {
    StudentNumber int      `json:"student_number"`
    Name          string   `json:"name"`
    Subjects      strslice `json:"subjects"`
}

This allows us to unmarshal the JSON data directly into the struct, with the Subjects field automatically split into individual strings:

json := `{"student_number":1234567, "name":"John Doe", "subjects":"Chemistry-Maths-History-Geography"}`
var s Student
if err := json.Unmarshal([]byte(json), &s); err != nil {
    panic(err)
}
fmt.Println(s) // Output: {1234567 John Doe [Chemistry Maths History Geography]}