Unmarshal JSON date string to BSON date in Golang

This should be easy but I'm not making much headway. Say I have JSON with a UTC date like this:

{
  "name": "Lex",
  "dob": "2022-11-01T06:30:30.639326208Z"
}

I'd like to insert it in a MongoDB collection. In Go, I do:

import (
	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
)

...

var doc any
// by the way what does the 2nd param do?  docs don't mention it
_ = bson.UnmarshalExtJSON(jsonBytes, false, &doc)
mongoCollection.InsertOne(context.TODO(), doc)

The insert works, but the dob field is just a string. What's the correct way to insert it as Date? My constraints are:

1. I cannot change the input JSON
2. I can't manually write the Mongo doc because I could be dealing with large unpredictable objects and won't know which fields/nested fields have a date string

Is this doable? Seems like there has to be a paved road for how to do this.

You could define one custom decoders of bsoncodec.ValueDecoder to decode the date string to time.Time, then register it through RegisterTypeDecoder

func dateTimeDecodeValue(dc bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
	ts, _ := vr.ReadString()
	t, err := time.Parse(time.RFC3339Nano, ts)
	if err != nil {
		return err
	} 

	val.Set(reflect.ValueOf(t))

	return nil
}

var tDateTime = reflect.TypeOf(time.Time{})

type TestDoc struct {
	Name string    `bson:"name"`
	Dob  time.Time `bson:"dob"`
}

func createCustomRegistry() *bsoncodec.RegistryBuilder {
	var primitiveCodecs bson.PrimitiveCodecs

	rb := bsoncodec.NewRegistryBuilder()
	bsoncodec.DefaultValueEncoders{}.RegisterDefaultEncoders(rb)
	bsoncodec.DefaultValueDecoders{}.RegisterDefaultDecoders(rb)
	rb.RegisterTypeDecoder(tDateTime, bsoncodec.ValueDecoderFunc(dateTimeDecodeValue))
	primitiveCodecs.RegisterPrimitiveCodecs(rb)
	return rb
}

Or to parse date string to BSON date sample codes

func dateTimeDecodeValue(dc bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
	ts, _ := vr.ReadString()
	t, err := time.Parse(time.RFC3339Nano, ts)
	if err != nil {
		val.SetString(ts)
	} else {
		val.Set(reflect.ValueOf(primitive.DateTime(t.UnixMilli())))
	}

	return nil
}

var tDateTime = reflect.TypeOf(primitive.DateTime(0))

type TestDoc struct {
	Name string             `bson:"name"`
	Dob  primitive.DateTime `bson:"dob"`
}

func createCustomRegistry() *bsoncodec.RegistryBuilder {
	var primitiveCodecs bson.PrimitiveCodecs

	rb := bsoncodec.NewRegistryBuilder()
	bsoncodec.DefaultValueEncoders{}.RegisterDefaultEncoders(rb)
	bsoncodec.DefaultValueDecoders{}.RegisterDefaultDecoders(rb)
	rb.RegisterTypeDecoder(tDateTime, bsoncodec.ValueDecoderFunc(dateTimeDecodeValue))
	primitiveCodecs.RegisterPrimitiveCodecs(rb)
	return rb
}

Then use this register through UnmarshalExtJSONWithRegistry

func main() {
	jsonBytes := []byte(`{
       "name": "ben",
       "dob": "2022-11-01T06:30:30.639326208Z"
    }`)

	var customRegistry = createCustomRegistry().Build()

	var doc TestDoc
	_ = bson.UnmarshalExtJSONWithRegistry(customRegistry, jsonBytes, false, &doc)
	fmt.Println(doc)
}

Full codes
<kbd>PLAYGROUND</kbd>

As for Canonical parameter please refer to doc

> Canonical Extended JSON - A string format based on the JSON standard that describes BSON documents. Canonical Extended JSON emphasizes type preservation at the expense of readability and interoperability.