Invalid merge of two map[string]interface{}

[dshylov]

Merge of

func main() {
	map1 := map[string]interface{}{
		"first": 1,
	}
	map2 := map[string]interface{}{
		"first": map[string]interface{}{
			"second" : 2,
		},
	}

	mergo.Merge(&map1, map2)

	fmt.Println(map1)
}

will returns

map[first:1]

[dshylov]

Result with mergo.WithOverride is the same

[birdayz]

i've just discovered the same issue. is this intended?

[dshylov]

@imdario any updates?

[0x1mason]

@dshylov From README You can only merge same-type structs with exported fields initialized as zero value of their type and same-types maps. You've got different types.

[darccio]

@0x1mason, actually, maps are more flexible. Both maps have the same type map[string]interface{} so they should count as "same-type maps" but I'll check it.

[Overbryd]

I found the same.
Triggered in my helm templating. Both are of type map[string]interface{}.
Helm uses Masterminds/sprig to provide merge functionality for their template values.

This breaks the idea and "contract" implemented here: https://github.com/Masterminds/sprig/blob/master/dict.go#L80
I will also create an issue on their side.

I whipped up an example that shows the unexpected behaviour:

package main

import (
  "fmt"
  "github.com/imdario/mergo"
)

func main() {
  map1 := map[string]interface{}{
    "deep": map[string]interface{}{
      "enabled": true,
    },
    "key": "default",
  }
  map2 := map[string]interface{}{
    "deep": map[string]interface{}{
      "enabled": false,
    },
    "key": "overwritten",
  }

  mergo.Merge(&map1, map2)

  fmt.Println(map1)
}

Expected output

map[deep:map[enabled:false] key:overwritten]

Actual output

$ go get -u github.com/imdario/mergo
map[deep:map[enabled:true] key:default]

[vdemeester]

@Overbryd if you use mergo.WithOverride in the Merge methods, it should work..

package main

import (
  "fmt"
  "github.com/imdario/mergo"
)

func main() {
  map1 := map[string]interface{}{
    "deep": map[string]interface{}{
      "enabled": true,
    },
    "key": "default",
  }
  map2 := map[string]interface{}{
    "deep": map[string]interface{}{
      "enabled": false,
    },
    "key": "overwritten",
  }

  mergo.Merge(&map1, map2)

  fmt.Println(map1)
}

This will print map[deep:map[enabled:false] key:overwritten]

[Overbryd]

@vdemeester thanks, I can confirm that mergo.Merge(&map1, map2, mergo.WithOverride) works as expected.

[darccio]

Solved fixing #143 by reverting PR #105