diff --git a/tests/update_has_one_test.go b/tests/update_has_one_test.go
index 28d5e52..1f39d83 100644
--- a/tests/update_has_one_test.go
+++ b/tests/update_has_one_test.go
@@ -40,6 +40,7 @@ package tests
 
 import (
 	"database/sql"
+	"errors"
 	"testing"
 
 	"time"
@@ -47,6 +48,7 @@ import (
 	. "github.com/oracle-samples/gorm-oracle/tests/utils"
 
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 	"gorm.io/gorm/utils/tests"
 )
 
@@ -128,6 +130,87 @@ func TestUpdateHasOne(t *testing.T) {
 		CheckPetSkipUpdatedAt(t, pet4, pet)
 	})
 
+	t.Run("ReplaceAssociation", func(t *testing.T) {
+		user := *GetUser("replace-has-one", Config{})
+
+		if err := DB.Create(&user).Error; err != nil {
+			t.Fatalf("errors happened when create user: %v", err)
+		}
+
+		acc1 := Account{AccountNumber: "first-account"}
+		user.Account = acc1
+
+		if err := DB.Save(&user).Error; err != nil {
+			t.Fatalf("errors happened when saving user with first account: %v", err)
+		}
+
+		acc2 := Account{AccountNumber: "second-account"}
+		user.Account = acc2
+		if err := DB.Session(&gorm.Session{FullSaveAssociations: true}).Save(&user).Error; err != nil {
+			t.Fatalf("errors happened when replacing association: %v", err)
+		}
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Account.AccountNumber != "second-account" {
+			t.Fatalf("expected replaced account to have AccountNumber 'second-account', got %v", result.Account.AccountNumber)
+		}
+	})
+
+	t.Run("ClearHasOneAssociation", func(t *testing.T) {
+		user := *GetUser("nullify-has-one", Config{})
+
+		if err := DB.Create(&user).Error; err != nil {
+			t.Fatalf("errors happened when create user: %v", err)
+		}
+
+		user.Account = Account{AccountNumber: "to-be-nullified"}
+		if err := DB.Save(&user).Error; err != nil {
+			t.Fatalf("errors happened when saving user: %v", err)
+		}
+
+		DB.Model(&user).Association("Account").Clear()
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Account.AccountNumber != "" {
+			t.Fatalf("expected account to be nullified/empty, got %v", result.Account.AccountNumber)
+		}
+	})
+
+	t.Run("ClearPolymorphicAssociation", func(t *testing.T) {
+		pet := Pet{Name: "clear-poly"}
+		pet.Toy = Toy{Name: "polytoy"}
+		DB.Create(&pet)
+
+		DB.Model(&pet).Association("Toy").Clear()
+
+		var pet2 Pet
+		DB.Preload("Toy").First(&pet2, pet.ID)
+		if pet2.Toy.Name != "" {
+			t.Fatalf("expected Toy cleared, got %v", pet2.Toy.Name)
+		}
+	})
+
+	t.Run("UpdateWithoutAssociation", func(t *testing.T) {
+		user := *GetUser("no-assoc-update", Config{})
+		if err := DB.Create(&user).Error; err != nil {
+			t.Fatalf("errors happened when create user: %v", err)
+		}
+		newName := user.Name + "-updated"
+		if err := DB.Model(&user).Update("name", newName).Error; err != nil {
+			t.Fatalf("errors happened when updating only parent: %v", err)
+		}
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Name != newName {
+			t.Fatalf("user name not updated as expected")
+		}
+		if result.Account.ID != 0 {
+			t.Fatalf("expected no Account associated, got ID %v", result.Account.ID)
+		}
+	})
+
 	t.Run("Restriction", func(t *testing.T) {
 		type CustomizeAccount struct {
 			gorm.Model
@@ -175,4 +258,201 @@ func TestUpdateHasOne(t *testing.T) {
 		tests.AssertEqual(t, account2.Number, number)
 		tests.AssertEqual(t, account2.Number2, cusUser.Account.Number2)
 	})
+
+	t.Run("AssociationWithoutPreload", func(t *testing.T) {
+		user := *GetUser("no-preload", Config{})
+		user.Account = Account{AccountNumber: "np-account"}
+		DB.Create(&user)
+
+		var result User
+		DB.First(&result, user.ID) // no preload
+		if result.Account.AccountNumber != "" {
+			t.Fatalf("expected Account field empty without preload, got %v", result.Account.AccountNumber)
+		}
+
+		var acc Account
+		DB.First(&acc, "\"user_id\" = ?", user.ID)
+		if acc.AccountNumber != "np-account" {
+			t.Fatalf("account not found as expected")
+		}
+	})
+
+	t.Run("SkipFullSaveAssociations", func(t *testing.T) {
+		user := *GetUser("skip-fsa", Config{})
+		user.Account = Account{AccountNumber: "skipfsa"}
+		DB.Create(&user)
+
+		user.Account.AccountNumber = "should-not-update"
+		if err := DB.Session(&gorm.Session{FullSaveAssociations: false}).Save(&user).Error; err != nil {
+			t.Fatalf("error saving with FSA false: %v", err)
+		}
+
+		var acc Account
+		DB.First(&acc, "\"user_id\" = ?", user.ID)
+		if acc.AccountNumber != "skipfsa" {
+			t.Fatalf("account should not have updated, got %v", acc.AccountNumber)
+		}
+	})
+
+	t.Run("HasOneZeroForeignKey", func(t *testing.T) {
+		now := time.Now()
+		user := User{Name: "zero-value-clear", Age: 18, Birthday: &now}
+		DB.Create(&user)
+
+		account := Account{AccountNumber: "to-clear", UserID: sql.NullInt64{Int64: int64(user.ID), Valid: true}}
+		DB.Create(&account)
+
+		account.UserID = sql.NullInt64{Int64: 0, Valid: false}
+		DB.Model(&account).Select("UserID").Updates(account)
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Account.AccountNumber != "" {
+			t.Fatalf("expected account cleared, got %v", result.Account.AccountNumber)
+		}
+	})
+
+	t.Run("PolymorphicZeroForeignKey", func(t *testing.T) {
+		pet := Pet{Name: "poly-zero"}
+		pet.Toy = Toy{Name: "polytoy-zero"}
+		DB.Create(&pet)
+
+		pet.Toy.OwnerID = ""
+		DB.Model(&pet.Toy).Select("OwnerID").Updates(&pet.Toy)
+
+		var pet2 Pet
+		DB.Preload("Toy").First(&pet2, pet.ID)
+		if pet2.Toy.Name != "" {
+			t.Fatalf("expected polymorphic association cleared, got %v", pet2.Toy.Name)
+		}
+	})
+
+	t.Run("InvalidForeignKey", func(t *testing.T) {
+		acc := Account{AccountNumber: "badfk", UserID: sql.NullInt64{Int64: 99999999, Valid: true}}
+		err := DB.Create(&acc).Error
+		if err == nil {
+			t.Fatalf("expected foreign key constraint error, got nil")
+		}
+	})
+
+	t.Run("UpdateWithSelectOmit", func(t *testing.T) {
+		user := *GetUser("select-omit", Config{})
+		user.Account = Account{AccountNumber: "selomit"}
+		DB.Create(&user)
+
+		user.Name = "selomit-updated"
+		user.Account.AccountNumber = "selomit-updated"
+		if err := DB.Select("Name").Omit("Account").Save(&user).Error; err != nil {
+			t.Fatalf("error on select/omit: %v", err)
+		}
+
+		var acc Account
+		DB.First(&acc, "\"user_id\" = ?", user.ID)
+		if acc.AccountNumber != "selomit" {
+			t.Fatalf("account should not update with Omit(Account), got %v", acc.AccountNumber)
+		}
+	})
+
+	t.Run("NestedUpdate", func(t *testing.T) {
+		user := *GetUser("nested-update", Config{})
+		user.Account = Account{AccountNumber: "nested"}
+		DB.Create(&user)
+
+		user.Name = "nested-updated"
+		user.Account.AccountNumber = "nested-updated"
+		if err := DB.Session(&gorm.Session{FullSaveAssociations: true}).Updates(&user).Error; err != nil {
+			t.Fatalf("nested update failed: %v", err)
+		}
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Name != "nested-updated" || result.Account.AccountNumber != "nested-updated" {
+			t.Fatalf("nested update didn't apply: %v / %v", result.Name, result.Account.AccountNumber)
+		}
+	})
+
+	t.Run("EmptyStructNoFullSave", func(t *testing.T) {
+		user := *GetUser("empty-nofsa", Config{})
+		user.Account = Account{AccountNumber: "keep"}
+		DB.Create(&user)
+
+		user.Account = Account{}
+		if err := DB.Save(&user).Error; err != nil {
+			t.Fatalf("save failed: %v", err)
+		}
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+		if result.Account.AccountNumber != "keep" {
+			t.Fatalf("account should not be cleared without FullSaveAssociations")
+		}
+	})
+
+	t.Run("DeleteParentCascade", func(t *testing.T) {
+		type AccountCascadeDelete struct {
+			gorm.Model
+			AccountNumber string
+			UserID        uint
+		}
+
+		type UserCascadeDelete struct {
+			gorm.Model
+			Name    string
+			Account AccountCascadeDelete `gorm:"foreignKey:UserID;constraint:OnDelete:CASCADE;"`
+		}
+
+		DB.Migrator().DropTable(&AccountCascadeDelete{}, &UserCascadeDelete{})
+		if err := DB.AutoMigrate(&UserCascadeDelete{}, &AccountCascadeDelete{}); err != nil {
+			t.Fatalf("failed to migrate: %v", err)
+		}
+
+		user := UserCascadeDelete{
+			Name: "delete-parent",
+			Account: AccountCascadeDelete{
+				AccountNumber: "cascade",
+			},
+		}
+
+		if err := DB.Create(&user).Error; err != nil {
+			t.Fatalf("failed to create user: %v", err)
+		}
+
+		if err := DB.Unscoped().Delete(&user).Error; err != nil {
+			t.Fatalf("delete parent failed: %v", err)
+		}
+
+		var acc AccountCascadeDelete
+		err := DB.First(&acc, "\"user_id\" = ?", user.ID).Error
+		if !errors.Is(err, gorm.ErrRecordNotFound) {
+			t.Fatalf("expected account deleted, got %v", acc)
+		}
+	})
+
+	t.Run("OmitAllAssociations", func(t *testing.T) {
+		user := *GetUser("omit-assoc", Config{})
+		user.Account = Account{AccountNumber: "original-child"}
+		if err := DB.Create(&user).Error; err != nil {
+			t.Fatalf("failed to create user: %v", err)
+		}
+
+		newName := "parent-updated"
+		user.Name = newName
+		user.Account.AccountNumber = "child-updated"
+
+		if err := DB.Model(&user).Omit(clause.Associations).Updates(user).Error; err != nil {
+			t.Fatalf("update with omit associations failed: %v", err)
+		}
+
+		var result User
+		DB.Preload("Account").First(&result, user.ID)
+
+		if result.Name != newName {
+			t.Fatalf("expected parent name updated to %v, got %v", newName, result.Name)
+		}
+
+		if result.Account.AccountNumber != "original-child" {
+			t.Fatalf("expected child to remain unchanged, got %v", result.Account.AccountNumber)
+		}
+	})
+
 }