How to Flatten Marshaled JSON Structs with Anonymous Members in Go?

Flattening Marshaled JSON Structs with Anonymous Members in Go

When marshaling JSON data in Go, anonymous members can introduce unexpected complexities. This article delves into a solution that addresses these challenges.

The Issue

Consider the following code:

<code class="go">type Hateoas struct {
    Anything
    Links map[string]string `json:"_links"`
}</code>

When Hateoas contains anonymous members, the JSON marshaler treats them as regular named fields, resulting in undesired nesting:

<code class="json">{
    "Anything": {
        "id": 123,
        "name": "James Dean"
    },
    "_links": {
        "self": "http://user/123"
    }
}</code>

Solution: Reflection-Based Flattening

To flatten the JSON structure and eliminate the extra nesting, we can utilize reflection:

<code class="go">subjectValue := reflect.Indirect(reflect.ValueOf(subject))
subjectType := subjectValue.Type()
for i := 0; i < subjectType.NumField(); i++ {
    field := subjectType.Field(i)
    name := subjectType.Field(i).Name
    out[field.Tag.Get("json")] = subjectValue.FieldByName(name).Interface()
}</code>

This loop iterates over the fields of the struct, extracts their JSON tag names, and maps them to a flattened map[string]interface{}. By using reflection, we avoid adding new named fields and retain the original flat structure.

Example

Here's an example of how to use this solution:

<code class="go">import "reflect"

func MarshalHateoas(subject interface{}) ([]byte, error) {
    links := make(map[string]string)
    out := make(map[string]interface{})
    subjectValue := reflect.Indirect(reflect.ValueOf(subject))
    subjectType := subjectValue.Type()
    for i := 0; i < subjectType.NumField(); i++ {
        field := subjectType.Field(i)
        name := subjectType.Field(i).Name
        out[field.Tag.Get("json")] = subjectValue.FieldByName(name).Interface()
    }
    switch s := subject.(type) {
    case *User:
        links["self"] = fmt.Sprintf("http://user/%d", s.Id)
    case *Session:
        links["self"] = fmt.Sprintf("http://session/%d", s.Id)
    }
    out["_links"] = links
    return json.MarshalIndent(out, "", "    ")
}</code>

With this improved MarshalHateoas function, the sample JSON output becomes:

<code class="json">{
    "id": 123,
    "name": "James Dean",
    "_links": {
        "self": "http://user/123"
    }
}</code>

Conclusion

By leveraging reflection, we can effectively flatten JSON structs with anonymous members and achieve the desired JSON structure without compromising data integrity. This solution provides a robust way to handle complex JSON serialization scenarios in Go.

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