How to Correctly MarshalJSON() for Embedded Structs with Custom Marshaling?

Idiomatic Way to Embed Struct with Custom MarshalJSON() Method

Problem:

How can we encode a struct with an embedded struct that has a custom MarshalJSON() method? The expected output is to preserve the custom marshaling of the embedded struct while also encoding the fields of the outer struct.

Example:

type Person struct {
    Name string `json:"name"`
}

type Employee struct {
    *Person
    JobRole string `json:"jobRole"`
}

When the embedded Person struct has a custom MarshalJSON() method:

func (p *Person) MarshalJSON() ([]byte, error) {
    return json.Marshal(struct {
        Name string `json:"name"`
    }{
        Name: strings.ToUpper(p.Name),
    })
}

Marshaling an Employee instance breaks, resulting in:

{"name": "BOB"}

Solution:

Instead of implementing MarshalJSON() on Person, create a new Name type that implements MarshalJSON(). Then, modify Person to use this Name type:

type Name string

func (n Name) MarshalJSON() ([]byte, error) {
    return json.Marshal(struct {
        Name string `json:"name"`
    }{
        Name: strings.ToUpper(string(n)),
    })
}

type Person struct {
    Name Name `json:"name"`
}

This allows the custom marshaling logic to be applied to the Name field while still encoding the Employee fields as expected.

Generic Solution:

For a more generic solution, implement MarshalJSON() on the outer struct. While methods on the inner type are promoted to the outer type, the outer type can then unmarshal the result into a generic structure like map[string]interface{} and add its own fields.

Example:

type Person struct {
    Name string `json:"name"`
}

type Employee struct {
    *Person
    JobRole string `json:"jobRole"`
}

func (e *Employee) MarshalJSON() ([]byte, error) {
    b, err := e.Person.MarshalJSON()
    if err != nil {
        return nil, err
    }

    var m map[string]interface{}
    if err := json.Unmarshal(b, &m); err != nil {
        return nil, err
    }

    m["jobRole"] = e.JobRole
    return json.Marshal(m)
}

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