Hello, I am re-posting this as an issue as it is an active need for me that is forcing me to fork out and apply PR's as patches to move forward.

Reference PR: mitchellh#294

PR Author @mthjs

Kudos.

I'm trying to convert a struct to a map, and can't figure out how to make it work.

Basically I have these structs:

type SQLStepResult struct {
	Queries []QueryResult `mapstructure:"queries"`
}

type QueryResult struct {
	Rows []map[string]any `mapstructure:"rows"`
}

and I want to convert SQLStepResult to:

{
  "queries": [
    {
      "rows": [
        // ...
      ]
    }
  ]
}

But I always end up with Rows instead of rows.

I tried using the undocumented mapstructure.RecursiveStructToMapHookFunc but it doesn't appear to do anything

Is there a solution?

Go playground for reference: https://go.dev/play/p/ZlTOw1mqnH5

Hello,
I'd like to add a decode hook for parsing url.URL. Would you accept a PR for it?

I would like to be able to decode to a map, with deep processing of the data, and collect metadata about the keys that were decoded. For example:

actual := map[string]interface{}
config := &mapstructure.DecoderConfig{
  Metadata:   &mapstructure.Metadata{},
  Result:     actual,
}
decoder, err := mapstructure.NewDecoder(config)

decoder.Decode(map[string]interface{}{
  "a": map[string]interface{}{
    "b": "value",
  },
})
assert.Equals(t,  []string{"[a]", "[a][b]"}, config.Metadata.Keys)

At the moment, the nested map (a) is simply referred to directly in the target map, and the .Metadata.Keys contains only [a].

Use Case

In the OpenTelemetry Collector we support a configuration like this:

receiver:
  http:
  grpc:

Here the http: entry has no fields, but its presence indicate that the user wants an HTTP server, with all standard defaults. If http: is missing than this server will not be started.

Problem
Today we're handling it with a custom hook for the receiver: which catches the scenario and applies the defaults. The hook is brittle as it depends on the exact YAML field name.

We've been experimenting with a custom "Optional" type that would allow to express this much cleaner:

type ReceiverConfig struct {
  HTTP optional.Optional[HTTPServerConfig] `mapstructure:"http"`
}

func DefaultConfig() *ReceiverConfig {
  return &ReceiverConfig{
    HTTP: optional.WithDefault(defaultHTTPConfig), // passing a function
  }
}

func defaultHTTPConfig() HTTPServerConfig { ... }

Here optional.WithDefault returns Optional that has no value, but if during unmarshaling it sees that there was a corresponding entry it first creates default value and then runs normal unmarshal on it.

The issue is that this approach doesn't work today because when mapstructure sees an empty value http: it just exists, without running the decode hook (which could've made the result non-empty).

Proposal

Could we add another config flag to allow not bailing early on empty values, at least to allow decoder hooks to run?

Using mapstructure for decoding into maps as target structure seems to be an odd edge case. There are various open issues, amongst them #12 or mitchellh#306 because MatchName is not applied.

As workaround I'm using peterbourgon/mergemap#9, but it would be nice if mapstructure could apply all features to maps, too.

Repost of The time.Time type is converted to empty map from github.com/mitchellh/mapstructure.

When using Decode, the fields of type time.Time from the input struct are being decoded to the zero value of time.Time, time.Date(1, time.January, 1, 0, 0, 0, 0, time.UTC), in the output struct.

Hi 👋
Thanks for taking over the project!

I've noticed in the past (and today I'm noticing again), that it's not immediately obvious to me how to use the DecodeHookFunc* family of types/ funcs. Both times, I ended up diving into the code to understand how they work from reading the tests and implementations of StringTo{stdLibType}HookFuncs.
The code is very easy to read, so this wasn't exactly a bad experience 😄 But: I'd still expect the GoDoc to contain all the info I need to use them correctly.

IMO they could really use an example and/ or a section in the package overview. Therefore, I suggest to

• add an example for DecodeHookFuncKind, DecodeHookFuncType and DecodeHookFuncValue
• add argument names to the signatures of DecodeHookFuncKind and DecodeHookFuncType (I don't think it's obvious in which order from and to are used, and the third interface{} argument can be cryptic without context)
• perhaps add a section to the top-level package doc string

Would anybody object if I created a PR to do this?

I receive data from multiple sources and the key's of the data are not always the same.

For example, one source may provide a map similar to
{"LAT": 1, "LON": 2}

While another source may provide a map similar to
{"RLT_FOO": 1, "RLN_BAR": 2}

It would be very helpful if I could simply supply multiple tags for a single field.

For example

type Foo struct {
  Lat float64 `mapstructure:"LAT,RLT_FOO"`
  Lon float64 `mapstructure:"LON,RLN_BAR"`
}

The tags could be resolved in order and the first one that matches could be used.
This would avoid having to run a re-keying operation on maps before running through the mapstructure decoder.

I know there is the MatchName function but that only works if the variations are derivable from a base fieldName, if each field could have one of multiple unique keys it becomes a pain.

Also, if I'm just flat out missing a feature that would make this easy... please let me know.
Thanks

package main

import (
	"fmt"
	"reflect"
	"time"

	"github.com/go-viper/mapstructure/v2"
)

type Person struct {
	Time time.Time `json:"time"`
	Name string    `json:"name"`
}

func main() {
	result := map[string]any{}
	input := Person{time.Now(), "testName"}

	decoder, errDecoder := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
		DecodeHook: func(f reflect.Type, t reflect.Type, data any) (any, error) {
			if value, ok := data.(*time.Time); ok {
				return value.Format(time.DateTime), nil
			}
			return data, nil
		},
		Result:  &result,
		TagName: "json",
	})

	if errDecoder != nil {
		panic(errDecoder)
	}

	err := decoder.Decode(input)
	if err != nil {
		panic(err)
	}

	fmt.Printf("%#v\n", result)
}

Here, I want to add the 'time' of the structure Convert the Time type to a string in the 'result', but it cannot be done. report errors:

panic: 'time' expected a map, got 'string'

goroutine 1 [running]:
main.main()
	C:/Users/78081/AppData/Roaming/JetBrains/GoLand2024.1/scratches/scratch.go:37 +0x1a6

Is there a solution?

There is currently a 2.0.0 alpha version which contains some useful fixes.

When I can expect a full 2.0.0 release, which is out of alpha?

Hello, thanks for having taken the time of forking https://github.com/mitchellh/mapstructure/

When using the original project noticed the dynamic decode hook was an expensive part of decoding due to it using reflect on the hook on each pass. Which can happens a lot due as soon as you have slices or nested struct.

I had opened this PR mitchellh#324 to pre compute the hook to typed method.

I think it would be easy to port to the fork, is that something that would interest you ? (In which case I would be interested in helping 😄 )

Fabrice