Polymorphic JSON field

[kpeu3i]

Hi, guys! I have a question about defining the JSON field which stores different structures depending on the entity type.

I know that I can define JSON like this:

field.JSON("data", Data{})

But in my case, I need to have something like that:

field.JSON("data", Data1{})
field.JSON("data", Data2{})
field.JSON("data", Data3{})

because different structs are stored inside the data field.

So, is it possible somehow to choose the correct data structure when scanning the JSON field based on another field value from the entity (e.g type)?

[a8m]

Hey @kpeu3i and thanks for opening this issue 👋

You can use a custom type that implements the json.Marshaler/json.Unmarshaler interfaces, store a field named Data (an interface type) and decide on the decoding stage what type to assign to it.

For example, for a field field.JSON("addr", Addr{}) that can hold multiple variants of "net addresses", I'll implement it as follows:

type Addr struct{ net.Addr }

func (a *Addr) UnmarshalJSON(data []byte) error {
	var types struct {
		TCP *net.TCPAddr `json:"tcp,omitempty"`
		UDP *net.UDPAddr `json:"udp,omitempty"`
	}
	if err := json.Unmarshal(data, &types); err != nil {
		return err
	}
	switch {
	case types.TCP != nil && types.UDP != nil:
		return errors.New("TCP and UDP addresses are mutually exclusive")
	case types.TCP != nil:
		a.Addr = types.TCP
	case types.UDP != nil:
		a.Addr = types.UDP
	}
	return nil
}

func (a Addr) MarshalJSON() ([]byte, error) {
	var types struct {
		TCP *net.TCPAddr `json:"tcp,omitempty"`
		UDP *net.UDPAddr `json:"udp,omitempty"`
	}
	switch a := a.Addr.(type) {
	case *net.TCPAddr:
		types.TCP = a
	case *net.UDPAddr:
		types.UDP = a
	default:
		return nil, fmt.Errorf("unsupported address type: %T", a)
	}
	return json.Marshal(types)
}

I also added an example PR for your use-case in this repository, please give it a look #2497

Closing, but please feel free to reopen or join our Discord server if you still need help with this.

[kpeu3i]

@a8m thanks for your response. I got your idea. But I'm trying to switch to Ent from an existing database and I cannot modify the data structure that is stored inside the data column.

In terms of your example, *net.TCPAddr and *net.UDPAddr are stored in my DB without wrapper struct types, so I cannot detect which struct should be used when unmarshaling bytes.

The only way to detect which struct should be used is the value of type column from the parent entity. So, I'm wondering is it possible somehow access other entity field values when scanning a value for the field? Seems like it should be possible inside the generated assignValues method.

[masseelch]

To add my idea to this:

You can use json.RawMessage as type in your schema and add a "getter" method in your entity that can read the type field and parse the JSON when accessed.

// in schema
field.Json("data", json.RawMessage{})

// With 1.18 you can use generics to return a union type
func (s MySchema) Data() Data1 | Data2 | Data3 { 
  switch s.Type {
    case: ....
  }
}

Once read hooks/interceptors are present in Ent, this can be done similar to the mutation hooks.

[a8m]

I think another nice solution for this is to use field.JSON("data", json.RawMessage{}) to store the field raw, and expose a method on the type T like ParseData() Data that can access the type and the data fields and can return you a parsed version of it. Same thing for Marshaling.

These fields can be added using custom templates or manually in an ent/custom.go file.

We can expose an API for calling custom functions in assignValues, but I need to think of a clean API for this. I'd expect something like this to reside on the field-level but the issue is that you need access to another field

[a8m]

hehe, @masseelch 😆

@hedwigz and I just discussed this and we both commented this at the same time :)

[kpeu3i]

Guys, thanks for the help I will follow your suggestion about json.RawMessage.