perf: merge nested preload query when using join (#6990)

* pref: merge nest preload query

* fix: preload test

callbacks/preload.go

@@ -123,8 +123,18 @@ func preloadEntryPoint(db *gorm.DB, joins []string, relationships *schema.Relati
 			if joined, nestedJoins := isJoined(name); joined {
 				switch rv := db.Statement.ReflectValue; rv.Kind() {
 				case reflect.Slice, reflect.Array:
-					for i := 0; i < rv.Len(); i++ {
+					if rv.Len() > 0 {
-						reflectValue := rel.Field.ReflectValueOf(db.Statement.Context, rv.Index(i))
+						reflectValue := rel.FieldSchema.MakeSlice().Elem()
+						reflectValue.SetLen(rv.Len())
+						for i := 0; i < rv.Len(); i++ {
+							frv := rel.Field.ReflectValueOf(db.Statement.Context, rv.Index(i))
+							if frv.Kind() != reflect.Ptr {
+								reflectValue.Index(i).Set(frv.Addr())
+							} else {
+								reflectValue.Index(i).Set(frv)
+							}
+						}
+
 						tx := preloadDB(db, reflectValue, reflectValue.Interface())
 						if err := preloadEntryPoint(tx, nestedJoins, &tx.Statement.Schema.Relationships, preloadMap[name], associationsConds); err != nil {
 							return err

tests/preload_test.go

@@ -1,14 +1,14 @@
 package tests_test
 
 import (
+	"context"
 	"encoding/json"
 	"regexp"
 	"sort"
 	"strconv"
 	"sync"
 	"testing"
-
+	"time"
-	"github.com/stretchr/testify/require"
 
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
@@ -337,7 +337,7 @@ func TestNestedPreloadWithNestedJoin(t *testing.T) {
 	DB.Migrator().DropTable(&Preload{}, &Join{}, &Nested{}, &Value{})
 	DB.Migrator().AutoMigrate(&Preload{}, &Join{}, &Nested{}, &Value{})
 
-	value := Value{
+	value1 := Value{
 		Name: "value",
 		Nested: Nested{
 			Preloads: []*Preload{
@@ -346,32 +346,98 @@ func TestNestedPreloadWithNestedJoin(t *testing.T) {
 			Join: Join{Value: "j1"},
 		},
 	}
-	if err := DB.Create(&value).Error; err != nil {
+	value2 := Value{
+		Name: "value2",
+		Nested: Nested{
+			Preloads: []*Preload{
+				{Value: "p3"}, {Value: "p4"}, {Value: "p5"},
+			},
+			Join: Join{Value: "j2"},
+		},
+	}
+
+	values := []*Value{&value1, &value2}
+	if err := DB.Create(&values).Error; err != nil {
 		t.Errorf("failed to create value, got err: %v", err)
 	}
 
 	var find1 Value
-	err := DB.Joins("Nested").Joins("Nested.Join").Preload("Nested.Preloads").First(&find1).Error
+	err := DB.Joins("Nested").Joins("Nested.Join").Preload("Nested.Preloads").First(&find1, value1.ID).Error
 	if err != nil {
 		t.Errorf("failed to find value, got err: %v", err)
 	}
-	AssertEqual(t, find1, value)
+	AssertEqual(t, find1, value1)
 
 	var find2 Value
 	// Joins will automatically add Nested queries.
-	err = DB.Joins("Nested.Join").Preload("Nested.Preloads").First(&find2).Error
+	err = DB.Joins("Nested.Join").Preload("Nested.Preloads").First(&find2, value2.ID).Error
 	if err != nil {
 		t.Errorf("failed to find value, got err: %v", err)
 	}
-	AssertEqual(t, find2, value)
+	AssertEqual(t, find2, value2)
 
 	var finds []Value
 	err = DB.Joins("Nested.Join").Joins("Nested").Preload("Nested.Preloads").Find(&finds).Error
 	if err != nil {
 		t.Errorf("failed to find value, got err: %v", err)
 	}
-	require.Len(t, finds, 1)
+	AssertEqual(t, len(finds), 2)
-	AssertEqual(t, finds[0], value)
+	AssertEqual(t, finds[0], value1)
+	AssertEqual(t, finds[1], value2)
+}
+
+func TestMergeNestedPreloadWithNestedJoin(t *testing.T) {
+	users := []User{
+		{
+			Name: "TestMergeNestedPreloadWithNestedJoin-1",
+			Manager: &User{
+				Name: "Alexis Manager",
+				Tools: []Tools{
+					{Name: "Alexis Tool 1"},
+					{Name: "Alexis Tool 2"},
+				},
+			},
+		},
+		{
+			Name: "TestMergeNestedPreloadWithNestedJoin-2",
+			Manager: &User{
+				Name: "Jinzhu Manager",
+				Tools: []Tools{
+					{Name: "Jinzhu Tool 1"},
+					{Name: "Jinzhu Tool 2"},
+				},
+			},
+		},
+	}
+
+	DB.Create(&users)
+
+	query := make([]string, 0)
+	sess := DB.Session(&gorm.Session{Logger: Tracer{
+		Logger: DB.Config.Logger,
+		Test: func(ctx context.Context, begin time.Time, fc func() (sql string, rowsAffected int64), err error) {
+			sql, _ := fc()
+			query = append(query, sql)
+		},
+	}})
+
+	var result []User
+	err := sess.
+		Joins("Manager").
+		Preload("Manager.Tools").
+		Where("users.name Like ?", "TestMergeNestedPreloadWithNestedJoin%").
+		Find(&result).Error
+
+	if err != nil {
+		t.Fatalf("failed to preload and find users: %v", err)
+	}
+
+	AssertEqual(t, result, users)
+	AssertEqual(t, len(query), 2) // Check preload queries are merged
+
+	if !regexp.MustCompile(`SELECT \* FROM .*tools.* WHERE .*IN.*`).MatchString(query[0]) {
+		t.Fatalf("Expected first query to preload manager tools, got: %s", query[0])
+	}
 }
 
 func TestEmbedPreload(t *testing.T) {