MarshalJSON not called

Question

I'm trying to customize the output of MarshalJSON, 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
    fmt.Println("here")
    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"`
        *AliasEdition
    }{
        Vault:        vaultValue,
        Online:       onlineValue,
        AliasEdition: (*Alias)(u),
    })
}

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

json.NewEncoder(w).Encode(EditionsMap)

Obviously EditionsMap is a Map of Editions structures:

var EditionsMap map[string]datamodel.Edition

The problem is that the MarshalJSON 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 1

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
// ...
json.NewEncoder(w).Encode(EditionsMap)

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.