Join with preload does not work as intended

go.mod

@@ -3,6 +3,7 @@ module gorm.io/playground
 go 1.20
 
 require (
+	github.com/stretchr/testify v1.8.1
 	gorm.io/driver/mysql v1.5.2
 	gorm.io/driver/postgres v1.5.2
 	gorm.io/driver/sqlite v1.5.3
@@ -12,6 +13,7 @@ require (
 )
 
 require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/go-sql-driver/mysql v1.7.1 // indirect
 	github.com/golang-sql/civil v0.0.0-20220223132316-b832511892a9 // indirect
 	github.com/golang-sql/sqlexp v0.1.0 // indirect
@@ -20,13 +22,17 @@ require (
 	github.com/jackc/pgx/v5 v5.4.3 // indirect
 	github.com/jinzhu/inflection v1.0.0 // indirect
 	github.com/jinzhu/now v1.1.5 // indirect
+	github.com/kr/text v0.2.0 // indirect
 	github.com/mattn/go-sqlite3 v1.14.17 // indirect
 	github.com/microsoft/go-mssqldb v1.5.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/rogpeppe/go-internal v1.12.0 // indirect
 	golang.org/x/crypto v0.14.0 // indirect
 	golang.org/x/mod v0.14.0 // indirect
 	golang.org/x/sys v0.14.0 // indirect
-	golang.org/x/text v0.13.0 // indirect
+	golang.org/x/text v0.14.0 // indirect
 	golang.org/x/tools v0.15.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 	gorm.io/datatypes v1.1.1-0.20230130040222-c43177d3cf8c // indirect
 	gorm.io/hints v1.1.0 // indirect
 	gorm.io/plugin/dbresolver v1.5.0 // indirect

main_test.go

@@ -2,19 +2,139 @@ package main
 
 import (
 	"testing"
+
+	"gorm.io/gorm/clause"
+
+	"github.com/stretchr/testify/assert"
 )
 
 // GORM_REPO: https://github.com/go-gorm/gorm.git
 // GORM_BRANCH: master
 // TEST_DRIVERS: sqlite, mysql, postgres, sqlserver
 
+type M1 struct {
+	ID string `gorm:"column:id; primaryKey; not null"`
+
+	M2   *M2
+	M2ID *string `gorm:"column:m2_id"`
+}
+
+type M2 struct {
+	ID string `gorm:"column:id; primaryKey; not null"`
+
+	M1   *M1
+	M1ID *string `gorm:"column:m1_id"`
+}
+
 func TestGORM(t *testing.T) {
-	user := User{Name: "jinzhu"}
+	DB.Migrator().DropTable(&M1{}, &M2{})
+	DB.Migrator().AutoMigrate(&M1{}, &M2{})
+
+	// Create
+
+	m1 := M1{
+		ID: "m1",
+	}
+	m2 := M2{
+		ID: "m2",
+	}
+
+	assert.NoError(t, DB.Create(&m1).Error)
+	assert.NoError(t, DB.Create(&m2).Error)
+
+	// Set foreign keys
+	m1.M2ID = &m2.ID
+	m2.M1ID = &m1.ID
+	assert.NoError(t, DB.Model(&m1).Updates(&m1).Error)
+	assert.NoError(t, DB.Model(&m2).Updates(&m2).Error)
+	m1.M2 = &m2
+	m2.M1 = &m1
+
+	var findM1 M1
+	assert.NoError(t, DB.First(&findM1, M1{ID: m1.ID}).Error)
+	assert.Equal(t, findM1.ID, m1.ID)
+	assert.Equal(t, *findM1.M2ID, m2.ID)
+	assert.Nil(t, findM1.M2) // No preload
 
-	DB.Create(&user)
+	var findM2 M2
+	assert.NoError(t, DB.First(&findM2, M2{ID: m2.ID}).Error)
+	assert.Equal(t, findM2.ID, m2.ID)
+	assert.Equal(t, *findM2.M1ID, m1.ID)
+	assert.Nil(t, findM2.M1) // No preload
 
-	var result User
-	if err := DB.First(&result, user.ID).Error; err != nil {
-		t.Errorf("Failed, got error: %v", err)
+	// Now using preload all
+
+	assert.NoError(t, DB.Preload(clause.Associations).First(&findM1, M1{ID: m1.ID}).Error)
+	assert.Equal(t, findM1.ID, m1.ID)
+	assert.Equal(t, *findM1.M2ID, m2.ID)
+	assert.Equal(t, findM1.M2.ID, m2.ID) // With preload
+
+	assert.NoError(t, DB.Preload(clause.Associations).First(&findM2, M2{ID: m2.ID}).Error)
+	assert.Equal(t, findM2.ID, m2.ID)
+	assert.Equal(t, *findM2.M1ID, m1.ID)
+	assert.Equal(t, findM2.M1.ID, m1.ID) // With preload
+
+	// Break things by adding new database entries
+	{
+		m1New := M1{
+			ID: "m1-new",
+		}
+		m2New := &M2{
+			ID: "m2-new",
+		}
+
+		assert.NoError(t, DB.Create(&m1New).Error)
+		assert.NoError(t, DB.Create(&m2New).Error)
+
+		// Set foreign keys
+		m1New.M2ID = &m2.ID
+		m2New.M1ID = &m1.ID
+		assert.NoError(t, DB.Model(&m1New).Updates(&m1New).Error)
+		assert.NoError(t, DB.Model(&m2New).Updates(&m2New).Error)
+		m1New.M2 = &m2
+		m2New.M1 = &m1
 	}
+
+	// Now the same query as above yields different results
+
+	assert.NoError(t, DB.Preload(clause.Associations).First(&findM1, M1{ID: m1.ID}).Error)
+	assert.Equal(t, findM1.ID, m1.ID)
+	assert.Equal(t, *findM1.M2ID, m2.ID)
+	assert.Equal(t, findM1.M2.ID, m2.ID) // With preload
+
+	assert.NoError(t, DB.Preload(clause.Associations).First(&findM2, M2{ID: m2.ID}).Error)
+	assert.Equal(t, findM2.ID, m2.ID)
+	assert.Equal(t, *findM2.M1ID, m1.ID)
+	assert.Equal(t, findM2.M1.ID, m1.ID) // With preload
+
+	// Optional: Break even more things
+	{
+		m1New2 := M1{
+			ID: "m1-new2",
+		}
+		m2New2 := &M2{
+			ID: "m2-new2",
+		}
+
+		assert.NoError(t, DB.Create(&m1New2).Error)
+		assert.NoError(t, DB.Create(&m2New2).Error)
+
+		// Set foreign keys
+		m1New2.M2ID = &m2.ID
+		m2New2.M1ID = &m1.ID
+		m1New2.M2 = &m2
+		m2New2.M1 = &m1
+		assert.NoError(t, DB.Model(&m1New2).Updates(&m1New2).Error)
+		assert.NoError(t, DB.Model(&m2New2).Updates(&m2New2).Error)
+	}
+
+	// As we not only set the foreign ID but also the foreign object before
+	// updating above, the find operation fails now (which is a bug).
+	// The two issued queries don't make sense due to the duplicate "id" condition:
+	// SELECT * FROM `m1` WHERE `m1`.`id` = "m1-new2" AND `m1`.`id` = "m1" ORDER BY `m1`.`id` LIMIT 1
+	// SELECT * FROM `m2` WHERE `m2`.`id` = "m2-new2" AND `m2`.`id` = "m2" ORDER BY `m2`.`id` LIMIT 1
+
+	assert.NoError(t, DB.First(&findM1, M1{ID: m1.ID}).Error)
+
+	assert.NoError(t, DB.First(&findM2, M2{ID: m2.ID}).Error)
 }