added WithOverrideEmptySlice config flag

issue125_test.go

@@ -0,0 +1,37 @@
+package mergo
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+var (
+	data = `{"FirstSlice":[], "SecondSlice": null}`
+)
+
+type settings struct {
+	FirstSlice  []string `json:"FirstSlice"`
+	SecondSlice []string `json:"SecondSlice"`
+}
+
+func TestIssue125MergeWithOverwrite(t *testing.T) {
+
+	defaultSettings := settings{
+		FirstSlice:  []string{},
+		SecondSlice: []string{},
+	}
+
+	var something settings
+	if err := json.Unmarshal([]byte(data), &something); err != nil {
+		t.Errorf("Error while Unmarshalling maprequest: %s", err)
+	}
+	if err := Merge(&something, defaultSettings, WithOverrideEmptySlice); err != nil {
+		t.Errorf("Error while merging: %s", err)
+	}
+	if something.FirstSlice == nil {
+		t.Error("Invalid merging first slice")
+	}
+	if something.SecondSlice == nil {
+		t.Error("Invalid merging second slice")
+	}
+}

merge.go

@@ -26,11 +26,12 @@ func hasExportedField(dst reflect.Value) (exported bool) {
 }
 
 type Config struct {
-	Overwrite               bool
-	AppendSlice             bool
-	TypeCheck               bool
-	Transformers            Transformers
-	overwriteWithEmptyValue bool
+	Overwrite                    bool
+	AppendSlice                  bool
+	TypeCheck                    bool
+	Transformers                 Transformers
+	overwriteWithEmptyValue      bool
+	overwriteSliceWithEmptyValue bool
 }
 
 type Transformers interface {
@@ -44,6 +45,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 	overwrite := config.Overwrite
 	typeCheck := config.TypeCheck
 	overwriteWithEmptySrc := config.overwriteWithEmptyValue
+	overwriteSliceWithEmptySrc := config.overwriteSliceWithEmptyValue
 	config.overwriteWithEmptyValue = false
 
 	if !src.IsValid() {
@@ -130,7 +132,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 						dstSlice = reflect.ValueOf(dstElement.Interface())
 					}
 
-					if (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
+					if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
 						if typeCheck && srcSlice.Type() != dstSlice.Type() {
 							return fmt.Errorf("cannot override two slices with different type (%s, %s)", srcSlice.Type(), dstSlice.Type())
 						}
@@ -159,7 +161,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		if !dst.CanSet() {
 			break
 		}
-		if (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
+		if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
 			dst.Set(src)
 		} else if config.AppendSlice {
 			if src.Type() != dst.Type() {
@@ -244,6 +246,11 @@ func WithOverride(config *Config) {
 	config.Overwrite = true
 }
 
+// WithOverride will make merge override empty dst slice with empty src slice.
+func WithOverrideEmptySlice(config *Config) {
+	config.overwriteSliceWithEmptyValue = true
+}
+
 // WithAppendSlice will make merge append slices instead of overwriting it.
 func WithAppendSlice(config *Config) {
 	config.AppendSlice = true