How Do I Properly Handle Nested Empty Structs When Marshaling to JSON in Go?

Handling Nested Empty Structs in JSON Marshaling in Go

Introduction

When using the encoding/json package in Go for marshalling structs to JSON, the ",omitempty" tag can be used to exclude empty fields from the resulting JSON. However, this tag may not behave as expected for nested structs.

Question

Consider the following example:

type ColorGroup struct {
    ID     int `json:",omitempty"`
    Name   string
    Colors []string
}

type Total struct {
    A ColorGroup `json:",omitempty"`
    B string    `json:",omitempty"`
}

group := Total{
    A: ColorGroup{},
}

json.Marshal(group)

In this scenario, the JSON output should only include the B field, since the A field is empty. However, the output still includes the A field with empty values ({"A": {"Name": "", "Colors": null}, "B": null}).

Answer

The documentation for json marshaling in Go states that struct fields are considered empty if they are:

• Nil pointers
• Zero values (e.g., false, 0)

In the provided example, group.A is an empty struct, not a nil pointer or a collection type (e.g., slice, map). Therefore, it's not treated as an empty value by the marshaler.

To achieve the desired behavior, one can use a pointer to the nested struct:

type ColorGroup struct {
    ID     int `json:",omitempty"`
    Name   string
    Colors []string
}

type Total struct {
    A *ColorGroup `json:",omitempty"`
    B string    `json:",omitempty"`
}

group := Total{
    B: "abc",
}

json.Marshal(group)

With this modification, the JSON output will include only the B field: {"B": "abc"}.

Note:

• If a pointer to a non-zero struct is passed, it will be included in the JSON output, even if it's empty.
• This solution can only be applied to nested structs. For non-nested empty structs, using the ",omitempty" tag without pointers will suffice.

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