fix: preload shouldn't overwrite the value of join (#6771)

* fix: preload shouldn't overwrite the value of join

* fix lint

* fix: join may automatically add nested query

callbacks/preload.go

@@ -3,6 +3,7 @@ package callbacks
 import (
 	"fmt"
 	"reflect"
+	"sort"
 	"strings"
 
 	"gorm.io/gorm"
@@ -82,27 +83,80 @@ func embeddedValues(embeddedRelations *schema.Relationships) []string {
 	return names
 }
 
-func preloadEmbedded(tx *gorm.DB, relationships *schema.Relationships, s *schema.Schema, preloads map[string][]interface{}, as []interface{}) error {
-	if relationships == nil {
-		return nil
+// preloadEntryPoint enters layer by layer. It will call real preload if it finds the right entry point.
+// If the current relationship is embedded or joined, current query will be ignored.
+//
+//nolint:cyclop
+func preloadEntryPoint(db *gorm.DB, joins []string, relationships *schema.Relationships, preloads map[string][]interface{}, associationsConds []interface{}) error {
+	preloadMap := parsePreloadMap(db.Statement.Schema, preloads)
+
+	// avoid random traversal of the map
+	preloadNames := make([]string, 0, len(preloadMap))
+	for key := range preloadMap {
+		preloadNames = append(preloadNames, key)
 	}
-	preloadMap := parsePreloadMap(s, preloads)
-	for name := range preloadMap {
-		if embeddedRelations := relationships.EmbeddedRelations[name]; embeddedRelations != nil {
-			if err := preloadEmbedded(tx, embeddedRelations, s, preloadMap[name], as); err != nil {
+	sort.Strings(preloadNames)
+
+	isJoined := func(name string) (joined bool, nestedJoins []string) {
+		for _, join := range joins {
+			if _, ok := relationships.Relations[join]; ok && name == join {
+				joined = true
+				continue
+			}
+			joinNames := strings.SplitN(join, ".", 2)
+			if len(joinNames) == 2 {
+				if _, ok := relationships.Relations[joinNames[0]]; ok && name == joinNames[0] {
+					joined = true
+					nestedJoins = append(nestedJoins, joinNames[1])
+				}
+			}
+		}
+		return joined, nestedJoins
+	}
+
+	for _, name := range preloadNames {
+		if relations := relationships.EmbeddedRelations[name]; relations != nil {
+			if err := preloadEntryPoint(db, joins, relations, preloadMap[name], associationsConds); err != nil {
 				return err
 			}
 		} else if rel := relationships.Relations[name]; rel != nil {
-			if err := preload(tx, rel, append(preloads[name], as), preloadMap[name]); err != nil {
-				return err
+			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
+				}
+			} else {
+				tx := db.Table("").Session(&gorm.Session{Context: db.Statement.Context, SkipHooks: db.Statement.SkipHooks})
+				tx.Statement.ReflectValue = db.Statement.ReflectValue
+				tx.Statement.Unscoped = db.Statement.Unscoped
+				if err := preload(tx, rel, append(preloads[name], associationsConds...), preloadMap[name]); err != nil {
+					return err
+				}
 			}
 		} else {
-			return fmt.Errorf("%s: %w (embedded) for schema %s", name, gorm.ErrUnsupportedRelation, s.Name)
+			return fmt.Errorf("%s: %w for schema %s", name, gorm.ErrUnsupportedRelation, db.Statement.Schema.Name)
 		}
 	}
 	return nil
 }
 
+func preloadDB(db *gorm.DB, reflectValue reflect.Value, dest interface{}) *gorm.DB {
+	tx := db.Session(&gorm.Session{Context: db.Statement.Context, NewDB: true, SkipHooks: db.Statement.SkipHooks, Initialized: true})
+	db.Statement.Settings.Range(func(k, v interface{}) bool {
+		tx.Statement.Settings.Store(k, v)
+		return true
+	})
+
+	if err := tx.Statement.Parse(dest); err != nil {
+		tx.AddError(err)
+		return tx
+	}
+	tx.Statement.ReflectValue = reflectValue
+	tx.Statement.Unscoped = db.Statement.Unscoped
+	return tx
+}
+
 func preload(tx *gorm.DB, rel *schema.Relationship, conds []interface{}, preloads map[string][]interface{}) error {
 	var (
 		reflectValue     = tx.Statement.ReflectValue

callbacks/query.go

@@ -3,7 +3,6 @@ package callbacks
 import (
 	"fmt"
 	"reflect"
-	"sort"
 	"strings"
 
 	"gorm.io/gorm"
@@ -254,7 +253,6 @@ func BuildQuerySQL(db *gorm.DB) {
 			}
 
 			db.Statement.AddClause(fromClause)
-			db.Statement.Joins = nil
 		} else {
 			db.Statement.AddClauseIfNotExists(clause.From{})
 		}
@@ -272,38 +270,23 @@ func Preload(db *gorm.DB) {
 			return
 		}
 
-		preloadMap := parsePreloadMap(db.Statement.Schema, db.Statement.Preloads)
-		preloadNames := make([]string, 0, len(preloadMap))
-		for key := range preloadMap {
-			preloadNames = append(preloadNames, key)
+		joins := make([]string, 0, len(db.Statement.Joins))
+		for _, join := range db.Statement.Joins {
+			joins = append(joins, join.Name)
 		}
-		sort.Strings(preloadNames)
 
-		preloadDB := db.Session(&gorm.Session{Context: db.Statement.Context, NewDB: true, SkipHooks: db.Statement.SkipHooks, Initialized: true})
-		db.Statement.Settings.Range(func(k, v interface{}) bool {
-			preloadDB.Statement.Settings.Store(k, v)
-			return true
-		})
-
-		if err := preloadDB.Statement.Parse(db.Statement.Dest); err != nil {
+		tx := preloadDB(db, db.Statement.ReflectValue, db.Statement.Dest)
+		if tx.Error != nil {
 			return
 		}
-		preloadDB.Statement.ReflectValue = db.Statement.ReflectValue
-		preloadDB.Statement.Unscoped = db.Statement.Unscoped
 
-		for _, name := range preloadNames {
-			if relations := preloadDB.Statement.Schema.Relationships.EmbeddedRelations[name]; relations != nil {
-				db.AddError(preloadEmbedded(preloadDB.Table("").Session(&gorm.Session{Context: db.Statement.Context, SkipHooks: db.Statement.SkipHooks}), relations, db.Statement.Schema, preloadMap[name], db.Statement.Preloads[clause.Associations]))
-			} else if rel := preloadDB.Statement.Schema.Relationships.Relations[name]; rel != nil {
-				db.AddError(preload(preloadDB.Table("").Session(&gorm.Session{Context: db.Statement.Context, SkipHooks: db.Statement.SkipHooks}), rel, append(db.Statement.Preloads[name], db.Statement.Preloads[clause.Associations]...), preloadMap[name]))
-			} else {
-				db.AddError(fmt.Errorf("%s: %w for schema %s", name, gorm.ErrUnsupportedRelation, db.Statement.Schema.Name))
-			}
-		}
+		db.AddError(preloadEntryPoint(tx, joins, &tx.Statement.Schema.Relationships, db.Statement.Preloads, db.Statement.Preloads[clause.Associations]))
 	}
 }
 
 func AfterQuery(db *gorm.DB) {
+	// clear the joins after query because preload need it
+	db.Statement.Joins = nil
 	if db.Error == nil && db.Statement.Schema != nil && !db.Statement.SkipHooks && db.Statement.Schema.AfterFind && db.RowsAffected > 0 {
 		callMethod(db, func(value interface{}, tx *gorm.DB) bool {
 			if i, ok := value.(AfterFindInterface); ok {

tests/preload_test.go

@@ -307,6 +307,63 @@ func TestNestedPreloadWithUnscoped(t *testing.T) {
 	CheckUserUnscoped(t, *user6, user)
 }
 
+func TestNestedPreloadWithNestedJoin(t *testing.T) {
+	type (
+		Preload struct {
+			ID       uint
+			Value    string
+			NestedID uint
+		}
+		Join struct {
+			ID       uint
+			Value    string
+			NestedID uint
+		}
+		Nested struct {
+			ID       uint
+			Preloads []*Preload
+			Join     Join
+			ValueID  uint
+		}
+		Value struct {
+			ID     uint
+			Name   string
+			Nested Nested
+		}
+	)
+
+	DB.Migrator().DropTable(&Preload{}, &Join{}, &Nested{}, &Value{})
+	DB.Migrator().AutoMigrate(&Preload{}, &Join{}, &Nested{}, &Value{})
+
+	value := Value{
+		Name: "value",
+		Nested: Nested{
+			Preloads: []*Preload{
+				{Value: "p1"}, {Value: "p2"},
+			},
+			Join: Join{Value: "j1"},
+		},
+	}
+	if err := DB.Create(&value).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
+	if err != nil {
+		t.Errorf("failed to find value, got err: %v", err)
+	}
+	AssertEqual(t, find1, value)
+
+	var find2 Value
+	// Joins will automatically add Nested queries.
+	err = DB.Joins("Nested.Join").Preload("Nested.Preloads").First(&find2).Error
+	if err != nil {
+		t.Errorf("failed to find value, got err: %v", err)
+	}
+	AssertEqual(t, find2, value)
+}
+
 func TestEmbedPreload(t *testing.T) {
 	type Country struct {
 		ID   int `gorm:"primaryKey"`