fix m2m nested preload

callback_query_preload.go

@@ -288,7 +288,7 @@ func (scope *Scope) handleManyToManyPreload(field *Field, conditions []interface
 	// assign find results
 	var (
 		indirectScopeValue = scope.IndirectValue()
-		fieldsSourceMap    = map[string]reflect.Value{}
+		fieldsSourceMap    = map[string][]reflect.Value{}
 		foreignFieldNames  = []string{}
 	)
 
@@ -301,13 +301,21 @@ func (scope *Scope) handleManyToManyPreload(field *Field, conditions []interface
 	if indirectScopeValue.Kind() == reflect.Slice {
 		for j := 0; j < indirectScopeValue.Len(); j++ {
 			object := indirect(indirectScopeValue.Index(j))
-			fieldsSourceMap[toString(getValueFromFields(object, foreignFieldNames))] = object.FieldByName(field.Name)
+			key := toString(getValueFromFields(object, foreignFieldNames))
+			fieldsSourceMap[key] = append(fieldsSourceMap[key], object.FieldByName(field.Name))
 		}
 	} else if indirectScopeValue.IsValid() {
-		fieldsSourceMap[toString(getValueFromFields(indirectScopeValue, foreignFieldNames))] = indirectScopeValue.FieldByName(field.Name)
+		key := toString(getValueFromFields(indirectScopeValue, foreignFieldNames))
+		fieldsSourceMap[key] = append(fieldsSourceMap[key], indirectScopeValue.FieldByName(field.Name))
 	}
-
 	for source, link := range linkHash {
-		fieldsSourceMap[source].Set(reflect.Append(fieldsSourceMap[source], link...))
+		for i, field := range fieldsSourceMap[source] {
+			//If not 0 this means Value is a pointer and we already added preloaded models to it
+			if fieldsSourceMap[source][i].Len() != 0 {
+				continue
+			}
+			field.Set(reflect.Append(fieldsSourceMap[source][i], link...))
+		}
+
 	}
 }

preload_test.go

@@ -1118,6 +1118,81 @@ func TestNestedManyToManyPreload3(t *testing.T) {
 	}
 }
 
+func TestNestedManyToManyPreload3ForStruct(t *testing.T) {
+	type (
+		Level1 struct {
+			ID    uint
+			Value string
+		}
+		Level2 struct {
+			ID      uint
+			Value   string
+			Level1s []Level1 `gorm:"many2many:level1_level2;"`
+		}
+		Level3 struct {
+			ID       uint
+			Value    string
+			Level2ID sql.NullInt64
+			Level2   Level2
+		}
+	)
+
+	DB.DropTableIfExists(&Level1{})
+	DB.DropTableIfExists(&Level2{})
+	DB.DropTableIfExists(&Level3{})
+	DB.DropTableIfExists("level1_level2")
+
+	if err := DB.AutoMigrate(&Level3{}, &Level2{}, &Level1{}).Error; err != nil {
+		t.Error(err)
+	}
+
+	level1Zh := Level1{Value: "zh"}
+	level1Ru := Level1{Value: "ru"}
+	level1En := Level1{Value: "en"}
+
+	level21 := Level2{
+		Value:   "Level2-1",
+		Level1s: []Level1{level1Zh, level1Ru},
+	}
+
+	level22 := Level2{
+		Value:   "Level2-2",
+		Level1s: []Level1{level1Zh, level1En},
+	}
+
+	wants := []*Level3{
+		{
+			Value:  "Level3-1",
+			Level2: level21,
+		},
+		{
+			Value:  "Level3-2",
+			Level2: level22,
+		},
+		{
+			Value:  "Level3-3",
+			Level2: level21,
+		},
+	}
+
+	for _, want := range wants {
+		if err := DB.Save(&want).Error; err != nil {
+			t.Error(err)
+		}
+	}
+
+	var gots []*Level3
+	if err := DB.Preload("Level2.Level1s", func(db *gorm.DB) *gorm.DB {
+		return db.Order("level1.id ASC")
+	}).Find(&gots).Error; err != nil {
+		t.Error(err)
+	}
+
+	if !reflect.DeepEqual(gots, wants) {
+		t.Errorf("got %s; want %s", toJSONString(gots), toJSONString(wants))
+	}
+}
+
 func TestNestedManyToManyPreload4(t *testing.T) {
 	type (
 		Level4 struct {