How to decode JSON object based on key prefix

Question

I'm wondering if it is possible to decode a JSON object into struct based on prefix.

For example, I may have two JSON objects like following:

{ "id teacher": "10" }

and

{ "id student": "20" }

Currently I have two structs:

type Teacher struct { Id string `json:"id teacher"` } type Student struct { Id string `json:"id student"` }

I'm wondering if it is possible to have one struct and decode based on the prefix of the string (for example, by regex id.*) assuming that there will be only one matching field.

Even if you did that, youd still have a struct of unknown type. What are you really trying to do here? — Zombo
– Zombo, Commented Jul 18, 2021 at 22:59
@StevenPenny My goal is to have one struct that can be used to decode both types of json. What do you mean by "struct of unknown type"? — Alex Xu
– Alex Xu, Commented Jul 19, 2021 at 5:09

Grokify · Accepted Answer · 2021-07-19 18:34:02Z

If I understand what you're trying to do, you want to create a single struct for say Person and have a single property Person.Id which can be populated by either id teacher or id student.

In native encoding/json syntax, you might expect to see something like the following, but Go doesn't work this way.

type Person struct { Id string `json:"id student|id teacher"` }

One approach is to implement a custom UnmarshalJSON function like follows. This example uses strings.Index but it's just as easy to use a regular expression.

type Person struct { Id string Role string } func (p *Person) UnmarshalJSON(data []byte) error { var v map[string]interface{} if err := json.Unmarshal(data, &v); err != nil { return err } for key, val := range v { if strings.Index(key, "id ") == 0 { p.Id = val.(string) p.Role = key[3:] } } return nil }

Here's a fully operational example:

https://play.golang.org/p/1-tzUYcd5rJ

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