preoload not allowd before count (#5023)

Co-authored-by: ningfei <accelerator314@outlook.com>

errors.go

@@ -39,4 +39,6 @@ var (
 	ErrInvalidValue = errors.New("invalid value, should be pointer to struct or slice")
 	// ErrInvalidValueOfLength invalid values do not match length
 	ErrInvalidValueOfLength = errors.New("invalid association values, length doesn't match")
+	// ErrPreloadNotAllowed preload is not allowed when count is used
+	ErrPreloadNotAllowed = errors.New("preload is not allowed when count is used")
 )

finisher_api.go

@@ -367,6 +367,10 @@ func (db *DB) Delete(value interface{}, conds ...interface{}) (tx *DB) {
 
 func (db *DB) Count(count *int64) (tx *DB) {
 	tx = db.getInstance()
+	if len(tx.Statement.Preloads) > 0 {
+		tx.AddError(ErrPreloadNotAllowed)
+		return
+	}
 	if tx.Statement.Model == nil {
 		tx.Statement.Model = tx.Statement.Dest
 		defer func() {

tests/count_test.go

@@ -144,4 +144,14 @@ func TestCount(t *testing.T) {
 	if err := DB.Model(&User{}).Where("name = ?", "count-4").Group("name").Count(&count11).Error; err != nil || count11 != 1 {
 		t.Fatalf("Count should be 3, but got count: %v err %v", count11, err)
 	}
+
+	var count12 int64
+	if err := DB.Table("users").
+		Where("name in ?", []string{user1.Name, user2.Name, user3.Name}).
+		Preload("Toys", func(db *gorm.DB) *gorm.DB {
+			return db.Table("toys").Select("name")
+		}).Count(&count12).Error; err != gorm.ErrPreloadNotAllowed {
+		t.Errorf("should returns preload not allowed error, but got %v", err)
+	}
+	
 }