From 24501a3c1a9de1c0116001e9c4c74d532872267a Mon Sep 17 00:00:00 2001
From: Richard Knop <risoknop@gmail.com>
Date: Sat, 9 Jul 2016 18:51:38 +0800
Subject: [PATCH] Fixed bug when preload duplicates has many related objects.

---
 callback_query_preload.go |  7 ++++
 preload_test.go           | 78 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/callback_query_preload.go b/callback_query_preload.go
index d9ec8bdd..c9bfa866 100644
--- a/callback_query_preload.go
+++ b/callback_query_preload.go
@@ -186,6 +186,13 @@ func (scope *Scope) handleHasManyPreload(field *Field, conditions []interface{})
 		for j := 0; j < indirectScopeValue.Len(); j++ {
 			object := indirect(indirectScopeValue.Index(j))
 			objectRealValue := getValueFromFields(object, relation.AssociationForeignFieldNames)
+			if j > 0 {
+				prevObject := indirect(indirectScopeValue.Index(j - 1))
+				prevObjectRealValue := getValueFromFields(prevObject, relation.AssociationForeignFieldNames)
+				if toString(prevObjectRealValue) == toString(objectRealValue) {
+					continue
+				}
+			}
 			if results, ok := preloadMap[toString(objectRealValue)]; ok {
 				f := object.FieldByName(field.Name)
 				f.Set(reflect.Append(f, results...))
diff --git a/preload_test.go b/preload_test.go
index da3ee38f..fd5b3af6 100644
--- a/preload_test.go
+++ b/preload_test.go
@@ -1509,6 +1509,84 @@ func TestNilPointerSlice2(t *testing.T) {
 	}
 }
 
+func TestPrefixedPreloadDuplication(t *testing.T) {
+	type (
+		Level4 struct {
+			ID       uint
+			Level3ID uint
+		}
+		Level3 struct {
+			ID      uint
+			Level4s []*Level4
+		}
+		Level2 struct {
+			ID       uint
+			Level3ID sql.NullInt64 `sql:"index"`
+			Level3   *Level3
+		}
+		Level1 struct {
+			ID       uint
+			Level2ID sql.NullInt64 `sql:"index"`
+			Level2   *Level2
+		}
+	)
+
+	DB.DropTableIfExists(new(Level3))
+	DB.DropTableIfExists(new(Level4))
+	DB.DropTableIfExists(new(Level2))
+	DB.DropTableIfExists(new(Level1))
+
+	if err := DB.AutoMigrate(new(Level3), new(Level4), new(Level2), new(Level1)).Error; err != nil {
+		t.Error(err)
+	}
+
+	lvl := new(Level3)
+	if err := DB.Save(lvl).Error; err != nil {
+		t.Error(err)
+	}
+
+	sublvl1 := &Level4{Level3ID: lvl.ID}
+	if err := DB.Save(sublvl1).Error; err != nil {
+		t.Error(err)
+	}
+	sublvl2 := &Level4{Level3ID: lvl.ID}
+	if err := DB.Save(sublvl2).Error; err != nil {
+		t.Error(err)
+	}
+
+	lvl.Level4s = []*Level4{sublvl1, sublvl2}
+
+	want1 := Level1{
+		Level2: &Level2{
+			Level3: lvl,
+		},
+	}
+	if err := DB.Save(&want1).Error; err != nil {
+		t.Error(err)
+	}
+
+	want2 := Level1{
+		Level2: &Level2{
+			Level3: lvl,
+		},
+	}
+	if err := DB.Save(&want2).Error; err != nil {
+		t.Error(err)
+	}
+
+	want := []Level1{want1, want2}
+
+	var got []Level1
+	err := DB.Preload("Level2.Level3.Level4s").Find(&got).Error
+	if err != nil {
+		t.Error(err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %s; want %s", toJSONString(got), toJSONString(want))
+	}
+}
+
 func toJSONString(v interface{}) []byte {
 	r, _ := json.MarshalIndent(v, "", "  ")
 	return r