基于类型密钥解组动态JSON

Question

I'm looking for a solution which DOESN'T involve introducing an additional "generic" field like Value, Data, etc. which would be a placeholder for the variant field.

I have a JSON spec which describes several large structs, which hold mostly simple values, but occasionally a value which is a struct itself, with a dynamic type depending on the value of a certain field.

For example, both these JSON documents should unmarshal to the same Go struct:

{ 
  "some_data": "foo",
  "dynamic_field": { "type": "A", "name": "Johnny" },
  "other_data": "bar"
}

and

{
  "some_data": "foo",
  "dynamic_field": { "type": "B", "address": "Somewhere" },
  "other_data": "bar"
}

The JSON structure is set, I can't change it.

The Go struct must look like this:

type BigStruct struct {
  SomeData     string    `json:"some_data"`
  DynamicField Something `json:"dynamic_field"`
  OtherData    string    `json:"other_data"`
}

The question is how to actually do it and what that Something type should be.

I've started by making it an interface:

type Something interface {
  GetType() string
}

And have several structs and funcs to go with it:

type BaseDynamicType struct {
  Type string `json:"type"`
}

type DynamicTypeA struct {
  BaseDynamicType
  Name string `json:"name"`
}

type DynamicTypeB struct {
  BaseDynamicType
  Address string `json:"address"`
}

func (d *BaseDynamicType) GetType() string {
  return d.Type
}

The reason is, that when I get an instance of BigStruct, I can do this:

switch big.DynamicField.GetType() {
  case "A": // do something with big.DynamicField cast to DynamicTypeA
  case "B": // do something with big.DynamicField cast to DynamicTypeB
}

However, then I got stuck - how could this arrangement work with UnmarshalJSON? I think that BigStruct should implement UnmarshalJSON which would somehow inspect the Type field of the dynamic_field and then based on it, make DynamicField either a DynamicTypeA or DynamicTypeB.

But how? One way which probably doesn't work because of recursion would be:

• Mark DynamicField as json:"-"
• Implement UnmarshalJSON for BigStruct
• unmarshal the JSON into a map[string]interface{} in the BigStruct's UnmarshalJSON,
• inspect the dynamic_field value in the map, manually construct either DynamicTypeA or DynamicTypeB
• unmarshal the same data again into BigStruct
• fixup the DynamicField with the manually created values

... but that will lead to infinite recursion in the 5th step when I try to unmarshal the data into a BigStruct which would call the same UnmarshalJSON function which is currently executing.

Answer 1

type BigStruct struct {
    SomeData     string      `json:"some_data"`
    DynamicField DynamicType `json:"dynamic_field"`
    OtherData    string      `json:"other_data"`
}

type DynamicType struct {
    Value interface{}
}

func (d *DynamicType) UnmarshalJSON(data []byte) error {
    var typ struct {
        Type string `json:"type"`
    }
    if err := json.Unmarshal(data, &typ); err != nil {
        return err
    }
    switch typ.Type {
    case "A":
        d.Value = new(TypeA)
    case "B":
        d.Value = new(TypeB)
    }
    return json.Unmarshal(data, d.Value)

}

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

type TypeB struct {
    Address string `json:"address"`
}

https://play.golang.com/p/oKMKQTdzp7s

---

If you don't want to, or can't, change the type of the DynamicField you can put the UnmarshalJSON method on the BigStruct and declare a temporary type to avoid recursion.

func (b *BigStruct) UnmarshalJSON(data []byte) error {
    var typ struct {
        DF struct {
            Type string `json:"type"`
        } `json:"dynamic_field"`
    }
    if err := json.Unmarshal(data, &typ); err != nil {
        return err
    }

    switch typ.DF.Type {
    case "A":
        b.DynamicField = new(DynamicTypeA)
    case "B":
        b.DynamicField = new(DynamicTypeB)
    }

    type tmp BigStruct // avoids infinite recursion
    return json.Unmarshal(data, (*tmp)(b))
}

https://play.golang.com/p/at5Okp3VU2u