fix: nested preload with join panic when find (#6877)

Author: black-06

callbacks/preload.go

@@ -121,10 +121,23 @@ func preloadEntryPoint(db *gorm.DB, joins []string, relationships *schema.Relati
 			}
 		} else if rel := relationships.Relations[name]; rel != nil {
 			if joined, nestedJoins := isJoined(name); joined {
-				reflectValue := rel.Field.ReflectValueOf(db.Statement.Context, db.Statement.ReflectValue)
-				tx := preloadDB(db, reflectValue, reflectValue.Interface())
-				if err := preloadEntryPoint(tx, nestedJoins, &tx.Statement.Schema.Relationships, preloadMap[name], associationsConds); err != nil {
-					return err
+				switch rv := db.Statement.ReflectValue; rv.Kind() {
+				case reflect.Slice, reflect.Array:
+					for i := 0; i < rv.Len(); i++ {
+						reflectValue := rel.Field.ReflectValueOf(db.Statement.Context, rv.Index(i))
+						tx := preloadDB(db, reflectValue, reflectValue.Interface())
+						if err := preloadEntryPoint(tx, nestedJoins, &tx.Statement.Schema.Relationships, preloadMap[name], associationsConds); err != nil {
+							return err
+						}
+					}
+				case reflect.Struct:
+					reflectValue := rel.Field.ReflectValueOf(db.Statement.Context, rv)
+					tx := preloadDB(db, reflectValue, reflectValue.Interface())
+					if err := preloadEntryPoint(tx, nestedJoins, &tx.Statement.Schema.Relationships, preloadMap[name], associationsConds); err != nil {
+						return err
+					}
+				default:
+					return gorm.ErrInvalidData
 				}
 			} else {
 				tx := db.Table("").Session(&gorm.Session{Context: db.Statement.Context, SkipHooks: db.Statement.SkipHooks})

tests/preload_test.go

@@ -8,6 +8,8 @@ import (
 	"sync"
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
 	. "gorm.io/gorm/utils/tests"
@@ -362,6 +364,14 @@ func TestNestedPreloadWithNestedJoin(t *testing.T) {
 		t.Errorf("failed to find value, got err: %v", err)
 	}
 	AssertEqual(t, find2, value)
+
+	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, finds[0], value)
 }
 
 func TestEmbedPreload(t *testing.T) {