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Wrong target type restriction when decoding struct to map #311

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mullerch opened this issue Oct 27, 2022 · 1 comment · May be fixed by #313
Open

Wrong target type restriction when decoding struct to map #311

mullerch opened this issue Oct 27, 2022 · 1 comment · May be fixed by #313

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@mullerch
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mullerch commented Oct 27, 2022

When the target map to decode the result to is strongly typed, the decoding process fails, even if the map type matches the source structure.

func TestDecode_structToTypedMap(t *testing.T) {
	type SourceChild struct {
		String string `mapstructure:"string"`
	}

	type SourceParent struct {
		Child SourceChild `mapstructure:"child"`
	}

	var target map[string]map[string]interface{} // This is the reason the test fails
	//var target map[string]interface{} // With this variant it works

	source := SourceParent{
		Child: SourceChild{
			String: "hello",
		},
	}

	if err := Decode(source, &target); err != nil {
		t.Fatalf("got error: %s", err)
	}

	expected := map[string]interface{}{
		"child": map[string]interface{}{
			"string": "hello",
		},
	}

	if !reflect.DeepEqual(target, expected) {
		t.Fatalf("bad: \nexpected: %#v\nresult: %#v", expected, target)
	}
}

Produces:

=== RUN   TestDecode_structToTypedMap
    mapstructure_test.go:2884: got error: cannot assign type 'mapstructure.SourceChild' to map value field of type 'map[string]interface {}'
--- FAIL: TestDecode_structToTypedMap (0.00s)

As the target sutructure matches the needs of the decoding process, I would expect this to work.

mapstructure version bf980b3
go version go1.18.7 linux/amd64

@mullerch
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mullerch commented Oct 31, 2022

As far as I understand, the issue is here : https://github.com/mitchellh/mapstructure/blob/main/mapstructure.go#L976

The type of the map field is not the same as the parent (owning) map. This only works if the type is generic (interface{}).

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