fix: Respect overwriteWithEmptySrc when merging maps

Fixes /issues/230.

issue230_test.go

@@ -0,0 +1,85 @@
+package mergo_test
+
+import (
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+var testDataM = []struct {
+	M1                     mapTest
+	M2                     mapTest
+	WithOverrideEmptyValue bool
+	ExpectedMap            map[int]int
+}{
+	{
+		M1: mapTest{
+			M: map[int]int{1: 1, 3: 3},
+		},
+		M2: mapTest{
+			M: map[int]int{1: 2, 2: 2},
+		},
+		WithOverrideEmptyValue: true,
+		ExpectedMap:            map[int]int{1: 1, 3: 3},
+	},
+	{
+		M1: mapTest{
+			M: map[int]int{1: 1, 3: 3},
+		},
+		M2: mapTest{
+			M: map[int]int{1: 2, 2: 2},
+		},
+		WithOverrideEmptyValue: false,
+		ExpectedMap:            map[int]int{1: 1, 2: 2, 3: 3},
+	},
+	{
+		M1: mapTest{
+			M: map[int]int{},
+		},
+		M2: mapTest{
+			M: map[int]int{1: 2, 2: 2},
+		},
+		WithOverrideEmptyValue: true,
+		ExpectedMap:            map[int]int{},
+	},
+	{
+		M1: mapTest{
+			M: map[int]int{},
+		},
+		M2: mapTest{
+			M: map[int]int{1: 2, 2: 2},
+		},
+		WithOverrideEmptyValue: false,
+		ExpectedMap:            map[int]int{1: 2, 2: 2},
+	},
+}
+
+func withOverrideEmptyValue(enable bool) func(*mergo.Config) {
+	if enable {
+		return mergo.WithOverwriteWithEmptyValue
+	}
+
+	return mergo.WithOverride
+}
+
+func TestMergeMapWithOverride(t *testing.T) {
+	t.Parallel()
+
+	for _, data := range testDataM {
+		err := mergo.Merge(&data.M2, data.M1, withOverrideEmptyValue(data.WithOverrideEmptyValue))
+		if err != nil {
+			t.Errorf("Error while merging %s", err)
+		}
+
+		if len(data.M2.M) != len(data.ExpectedMap) {
+			t.Errorf("Got %d elements in map, but expected %d", len(data.M2.M), len(data.ExpectedMap))
+			return
+		}
+
+		for i, val := range data.M2.M {
+			if val != data.ExpectedMap[i] {
+				t.Errorf("Expected value: %d, but got %d while merging map", data.ExpectedMap[i], val)
+			}
+		}
+	}
+}

merge.go

@@ -205,6 +205,16 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 				dst.SetMapIndex(key, srcElement)
 			}
 		}
+
+		// Ensure that all keys in dst are deleted if they are not in src.
+		if overwriteWithEmptySrc {
+			for _, key := range dst.MapKeys() {
+				srcElement := src.MapIndex(key)
+				if !srcElement.IsValid() {
+					dst.SetMapIndex(key, reflect.Value{})
+				}
+			}
+		}
 	case reflect.Slice:
 		if !dst.CanSet() {
 			break