feat: Faker should preserve previous values, if set (#1429)

This helps simplify codegen/mockgen scenarios, where some enums aren't
faked correctly. This way the template struct can have correct enums in
a specific field, while faking other fields. Used in stripe plugin.

Co-authored-by: Kemal Hadimli <[email protected]>

Author: disq

faker/faker.go

@@ -48,11 +48,19 @@ func (f faker) getFakedValue(a any) (reflect.Value, error) {
 			ft := time.Now().Add(time.Duration(rand.Int63()))
 			return reflect.ValueOf(ft), nil
 		default:
+			cur := reflect.ValueOf(a)
+
 			v := reflect.New(t).Elem()
 			for i := 0; i < v.NumField(); i++ {
 				if !v.Field(i).CanSet() {
 					continue // to avoid panic to set on unexported field in struct
 				}
+				curField := cur.Field(i)
+				if curField.IsValid() && !curField.IsZero() {
+					v.Field(i).Set(curField) // preserve non-empty field values
+					continue
+				}
+
 				val, err := f.getFakedValue(v.Field(i).Interface())
 				if err != nil {
 					if err == errEFaceNotAllowed { // skip empty interfaces

faker/faker_test.go

@@ -51,6 +51,22 @@ func TestFakerWithCustomType(t *testing.T) {
 	assert.NotEmpty(t, a.D)
 }
 
+func TestFakerWithCustomTypePreserve(t *testing.T) {
+	a := fakerStructWithCustomType{
+		A: "A",
+		B: map[string]customType{"b": "B"},
+		C: map[customType]string{"c": "C"},
+		D: []customType{"D", "D2"},
+	}
+	if err := FakeObject(&a); err != nil {
+		t.Fatal(err)
+	}
+	assert.EqualValues(t, "A", a.A)
+	assert.EqualValues(t, map[string]customType{"b": "B"}, a.B)
+	assert.EqualValues(t, map[customType]string{"c": "C"}, a.C)
+	assert.EqualValues(t, []customType{"D", "D2"}, a.D)
+}
+
 type complexType struct {
 	IPAddress      net.IP
 	IPAddresses    []net.IP