From 5c14736b1926461a5e9190b16c3a44ba37d13af8 Mon Sep 17 00:00:00 2001
From: AgentGoose32 <agentgoose32@gmail.com>
Date: Tue, 28 Apr 2026 18:15:14 -0400
Subject: [PATCH] Support struct tags for map keys

---
 issue255_test.go | 41 +++++++++++++++++++++++++++++++++++++++++
 map.go           |  9 +++++++++
 merge.go         |  8 ++++++++
 3 files changed, 58 insertions(+)
 create mode 100644 issue255_test.go

diff --git a/issue255_test.go b/issue255_test.go
new file mode 100644
index 0000000..7e359cb
--- /dev/null
+++ b/issue255_test.go
@@ -0,0 +1,41 @@
+package mergo_test
+
+import (
+	"testing"
+
+	"dario.cat/mergo"
+)
+
+type issue255Person struct {
+	FirstName string `json:"first_name"`
+	LastName  string `json:"last_name,omitempty"`
+	Password  string `json:"-"`
+	Age       int
+}
+
+func TestIssue255MapStructToMapWithTaggedKeys(t *testing.T) {
+	person := issue255Person{
+		FirstName: "Ada",
+		LastName:  "Lovelace",
+		Password:  "secret",
+		Age:       36,
+	}
+
+	actual := map[string]interface{}{}
+	if err := mergo.Map(&actual, person, mergo.WithMapKeyTag("json")); err != nil {
+		t.Fatal(err)
+	}
+
+	if actual["first_name"] != "Ada" {
+		t.Fatalf("expected first_name key to be mapped from tag, got %#v", actual)
+	}
+	if actual["last_name"] != "Lovelace" {
+		t.Fatalf("expected tag name to be used, got %#v", actual)
+	}
+	if actual["password"] != "secret" {
+		t.Fatalf("expected '-' tag to fall back to default field name, got %#v", actual)
+	}
+	if actual["age"] != 36 {
+		t.Fatalf("expected untagged field to fall back to default field name, got %#v", actual)
+	}
+}
diff --git a/map.go b/map.go
index 759b4f7..2d64228 100644
--- a/map.go
+++ b/map.go
@@ -11,6 +11,7 @@ package mergo
 import (
 	"fmt"
 	"reflect"
+	"strings"
 	"unicode"
 	"unicode/utf8"
 )
@@ -58,6 +59,14 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 			}
 			fieldName := field.Name
 			fieldName = changeInitialCase(fieldName, unicode.ToLower)
+			if config.mapKeyTag != "" {
+				if taggedName := field.Tag.Get(config.mapKeyTag); taggedName != "" {
+					taggedName, _, _ = strings.Cut(taggedName, ",")
+					if taggedName != "" && taggedName != "-" {
+						fieldName = taggedName
+					}
+				}
+			}
 			if _, ok := dstMap[fieldName]; !ok || (!isEmptyValue(reflect.ValueOf(src.Field(i).Interface()), !config.ShouldNotDereference) && overwrite) || config.overwriteWithEmptyValue {
 				dstMap[fieldName] = src.Field(i).Interface()
 			}
diff --git a/merge.go b/merge.go
index 5488338..0cba952 100644
--- a/merge.go
+++ b/merge.go
@@ -46,6 +46,7 @@ type Config struct {
 	overwriteWithEmptyValue      bool
 	overwriteSliceWithEmptyValue bool
 	sliceDeepCopy                bool
+	mapKeyTag                    string
 }
 
 type Transformers interface {
@@ -367,6 +368,13 @@ func WithSliceDeepCopy(config *Config) {
 	config.Overwrite = true
 }
 
+// WithMapKeyTag uses the named struct tag value as the destination map key when mapping a struct to map.
+func WithMapKeyTag(tag string) func(*Config) {
+	return func(config *Config) {
+		config.mapKeyTag = tag
+	}
+}
+
 func merge(dst, src interface{}, opts ...func(*Config)) error {
 	if dst != nil && reflect.ValueOf(dst).Kind() != reflect.Ptr {
 		return ErrNonPointerArgument