Ivan Ivan - 11 months ago 59
JSON Question

MarshalJSON not called

I'm trying to customize the output of

, using the interface:

func (m *RawMessage) MarshalJSON() ([]byte, error)

I followed that tutorial: http://choly.ca/post/go-json-marshalling/

My purpose is removing replace one of the fields with true/false (if set or not), so I ended up writing that function:

func (u *Edition) MarshalJSON() ([]byte, error) {
var vaultValue bool
vaultValue = true
var onlineValue bool
vaultValue = false
if u.Vault == nil {
vaultValue = false
if u.Online == nil {
onlineValue = false
type AliasEdition Edition
return json.Marshal(&struct {
Vault bool `json:"vault,omitempty"`
Online bool `json:"online,omitempty"`
Vault: vaultValue,
Online: onlineValue,
AliasEdition: (*Alias)(u),

The JSON is created from a map with the following instruction:


is a Map of
s structures:

var EditionsMap map[string]datamodel.Edition

The problem is that the
function apparently is never called.

Probably I'm doing something wrong, but I cannot understand what is the problem, my understanding is that I just need to implement that function in order to get it called.

Answer Source

This is because you declared the Edition.MarshalJSON() method with pointer receiver:

func (u *Edition) MarshalJSON() ([]byte, error)

And you try to marshal non-pointer values (your map contains datamodel.Edition values):

var EditionsMap map[string]datamodel.Edition
// ...

Methods with pointer receiver are not part of the method set of the corresponding non-pointer type. The method set of type datamodel.Edition does not contain the method MarshalJSON().

Spec: Method sets:

A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T).

Try to marshal pointer values, define your map to contain pointers:

var EditionsMap map[string]*datamodel.Edition
// ...
if err := json.NewEncoder(w).Encode(EditionsMap); err != nil {
    panic(err) // HANDLE error somehow, do not omit it like in your example!

Values of the pointer type *Edition does have a method MarshalJSON() which will be called properly by the json package. Try a working example of this on the Go Playground.

Another option would be to define the Edition.MarshalJSON() method with value receiver:

func (u Edition) MarshalJSON() ([]byte, error)

And this way it would work no matter if you marshal pointer or non-pointer values, as the methods with value receiver are part of the method set of both the Edition type and the corresponding *Edition pointer type. Try a working example of this variant on the Go Playground.