bool custom Transformer for non zero-value false

[mberlanda]

hi @iamdario,

I recently introduced mergo on one of our projects.

I came up with an issue very similar to #24 :

import (
  "testing"
  "github.com/imdario/mergo"
  "github.com/stretchr/testify/assert"
)
func TestBoolean(t *testing.T) {
  type Foo struct {
    Bar bool `json:"bar"`
  }

  f1 := Foo{Bar: true}
  f2 := Foo{Bar: false}

  mergo.Merge(&f2, f1)
  assert.False(t, f2.Bar)
}

Do you think it could be handled with some custom transformers?

[hectorj2f]

Is there any progress on this ? I am facing the same issue

[luisdavim]

I think this is the same issue reported in Praqma/helmsman#137

[svyotov]

As far as I can see this is because of https://github.com/imdario/mergo/blob/9316a62528ac99aaecb4e47eadd6dc8aa6533d58/mergo.go#L36-L59
Merging Boolean and Numeric values that are false or zero are considered empty thus replaced.

[github4jiawen]

This can be accomplished with custom bool and transformer feature

package main

import (
	"encoding/json"
	"log"
	"reflect"

	"github.com/imdario/mergo"
)

type BoolInSetting struct {
	Set   bool
	Value bool
}

func (b *BoolInSetting) UnmarshalJSON(data []byte) error {
	b.Set = true

	err := json.Unmarshal(data, &b.Value)
	return err
}

type BoolInSettingTransformer struct {
}

func (t BoolInSettingTransformer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error {
	if (typ == reflect.TypeOf(BoolInSetting{})) {
		return func(dst, src reflect.Value) error {
			if dst.CanSet() && src.FieldByName("Set").Bool() {
				dst.Set(src)
			}
			return nil
		}
	}
	return nil
}

type ConfigWithBool struct {
	BooleanField BoolInSetting `json:"BooleanField"`
}

func main() {
	var trusy ConfigWithBool
	var falsy ConfigWithBool
	var err error
	err = json.Unmarshal([]byte(`{"BooleanField": true}`), &trusy)
	if err != nil {
		log.Fatalln(err)
	}
	err = json.Unmarshal([]byte(`{"BooleanField": false}`), &falsy)
	if err != nil {
		log.Fatalln(err)
	}
	err = mergo.Merge(&trusy, falsy, mergo.WithTransformers(BoolInSettingTransformer{}))
	if err != nil {
		log.Fatalln(err)
	}
	log.Printf("%+v", trusy)
}

[vilipwong]

Hello, is mergo.WithOverride or mergo.WithOverwriteWithEmptyValue suppose to solve this ?

I faced the same issue with string and boolean value, using mergo.WithOverwriteWithEmptyValue e.g:

p1 := map[string]interface{}{
	"A": 3, "B": "note", "C": true,
}
p2 := map[string]interface{}{
	"B": "", "C": false,
}
mergo.Merge(&p1, &p2, mergo.WithOverwriteWithEmptyValue)

Expected result:
{A:3 B:"" C:false}

Actual result:
{A:3 B:"note" C:true}

[darccio]

Ok, let's see. The OP's example expects f2.Bar to be false, and it is the destination variable, so it's always truthy.

I rewrote the example with more meaningful variable names, as it was misleading me to the opposite direction:

import (
  "testing"
  "github.com/imdario/mergo"
  "github.com/stretchr/testify/assert"
)
func TestBoolean(t *testing.T) {
  type Foo struct {
    Bar bool `json:"bar"`
  }

  f1 := Foo{Bar: true}
  f2 := Foo{Bar: false}

  mergo.Merge(&f2, f1)
  assert.False(t, f2.Bar)
}

Your intention, from what I understood, is to keep the false value as is. Not sure why as I'm missing context here. I'm going to tag along and restate what I said in #24:

This is the expected behavior. As stated in README:

Mergo is intended to assign only zero value fields on destination with source value.

Provided this, your example won't work with the current implementation. If you need to protect false values, you must follow @github4jiawen example. Transformers are a simple way to filter the main behavior.

About your case, @vilipwong, I pushed another test to check it out, and I confirm you that mergo.WithOverwriteWithEmptyValue will help you. Take into account that you have the opposite problem, as you want to override a truthy value with false.

[vilipwong]

I see. i was using release v0.3.9, it works when i go get @master. Any reason why its not released ?
@imdario

[darccio]

@vilipwong I want to do the release with some more fixes from v0.3.9 related bugs.

[TomOperator]

For those of you who need it, @Pothulapati built a fork that solves this here.