diff --git a/operator.go b/operator.go
index 7e2653c..e647512 100644
--- a/operator.go
+++ b/operator.go
@@ -5,24 +5,26 @@ import (
 )
 
 // ContextOperator inject and extract Tx from context.Context.
-type ContextOperator[B any, T Tx] struct {
-	beginner *B
+//
+// Default ContextOperator uses comparable key for context.Context value operation.
+type ContextOperator[K comparable, T Tx] struct {
+	key K
 }
 
 // NewContextOperator returns new ContextOperator.
-func NewContextOperator[B any, T Tx](b *B) *ContextOperator[B, T] {
-	return &ContextOperator[B, T]{
-		beginner: b,
+func NewContextOperator[K comparable, T Tx](key K) *ContextOperator[K, T] {
+	return &ContextOperator[K, T]{
+		key: key,
 	}
 }
 
 // Inject returns new context.Context contains Tx as value.
-func (p *ContextOperator[B, T]) Inject(ctx context.Context, tx T) context.Context {
-	return context.WithValue(ctx, p.beginner, tx)
+func (p *ContextOperator[K, T]) Inject(ctx context.Context, tx T) context.Context {
+	return context.WithValue(ctx, p.key, tx)
 }
 
 // Extract returns Tx extracted from context.Context.
-func (p *ContextOperator[B, T]) Extract(ctx context.Context) (T, bool) {
-	c, ok := ctx.Value(p.beginner).(T)
+func (p *ContextOperator[K, T]) Extract(ctx context.Context) (T, bool) {
+	c, ok := ctx.Value(p.key).(T)
 	return c, ok
 }
diff --git a/stdlib/transactor.go b/stdlib/transactor.go
index 51a06bc..48f7284 100644
--- a/stdlib/transactor.go
+++ b/stdlib/transactor.go
@@ -25,7 +25,7 @@ type dbWrapper struct {
 }
 
 // BeginTx starts a transaction.
-func (db *dbWrapper) BeginTx(ctx context.Context, opts ...oniontx.Option[*sql.TxOptions]) (*txWrapper, error) {
+func (db dbWrapper) BeginTx(ctx context.Context, opts ...oniontx.Option[*sql.TxOptions]) (*txWrapper, error) {
 	var txOptions sql.TxOptions
 	for _, opt := range opts {
 		opt.Apply(&txOptions)
@@ -58,12 +58,12 @@ type Transactor struct {
 // NewTransactor returns new Transactor.
 func NewTransactor(db *sql.DB) *Transactor {
 	var (
-		base       = &dbWrapper{DB: db}
+		base       = dbWrapper{DB: db}
 		operator   = oniontx.NewContextOperator[*dbWrapper, *txWrapper](&base)
 		transactor = Transactor{
 			operator: operator,
 			transactor: oniontx.NewTransactor[*dbWrapper, *txWrapper, *sql.TxOptions](
-				base,
+				&base,
 				operator,
 			),
 		}
diff --git a/transactor_test.go b/transactor_test.go
index ca39c5b..57319f4 100644
--- a/transactor_test.go
+++ b/transactor_test.go
@@ -10,16 +10,51 @@ import (
 
 func Test_CtxOperator(t *testing.T) {
 	t.Run("success_extract_committer", func(t *testing.T) {
-		var (
-			ctx = context.Background()
-			c   = committerMock{}
-			b   = &beginnerMock[*committerMock, any]{}
-			o   = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-		)
-		ctx = o.Inject(ctx, &c)
-		extracted, ok := o.Extract(ctx)
-		assertTrue(t, ok)
-		assertTrue(t, extracted == &c)
+		t.Run("extract_pointer", func(t *testing.T) {
+			var (
+				ctx = context.Background()
+				c   = committerMock{}
+				b   = beginnerMock[*committerMock, any]{}
+				o   = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			)
+			ctx = o.Inject(ctx, &c)
+			extracted, ok := o.Extract(ctx)
+			assertTrue(t, ok)
+			assertTrue(t, extracted == &c)
+		})
+		t.Run("extract_value", func(t *testing.T) {
+			var (
+				ctx = context.Background()
+				c   = committerValueMock{
+					committer: &committerMock{},
+				}
+				b = beginnerValueMock[committerValueMock, any]{
+					beginner: &beginnerMock[committerValueMock, any]{},
+				}
+				o = NewContextOperator[beginnerValueMock[committerValueMock, any], committerValueMock](b)
+			)
+			ctx = o.Inject(ctx, c)
+			extracted, ok := o.Extract(ctx)
+			assertTrue(t, ok)
+			assertTrue(t, extracted == c)
+		})
+		t.Run("extract_nil_value", func(t *testing.T) {
+			var (
+				ctx = context.Background()
+				c   = committerValueMock{
+					committer: nil,
+				}
+				b = beginnerValueMock[committerValueMock, any]{
+					beginner: nil,
+				}
+				o = NewContextOperator[beginnerValueMock[committerValueMock, any], committerValueMock](b)
+			)
+			ctx = o.Inject(ctx, c)
+			extracted, ok := o.Extract(ctx)
+			assertTrue(t, ok)
+			assertTrue(t, extracted == c)
+		})
+
 	})
 }
 
@@ -36,7 +71,7 @@ func Test_Transactor(t *testing.T) {
 					return nil
 				},
 			}
-			b = &beginnerMock[*committerMock, any]{
+			b = beginnerMock[*committerMock, any]{
 				beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 					beginnerCalled = true
 					assertTrue(t, opts == nil)
@@ -44,7 +79,7 @@ func Test_Transactor(t *testing.T) {
 				},
 			}
 			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 		)
 		err := tr.WithinTx(ctx, func(ctx context.Context) error {
 			tx, ok := tr.TryGetTx(ctx)
@@ -67,7 +102,7 @@ func Test_Transactor(t *testing.T) {
 					return nil
 				},
 			}
-			b = &beginnerMock[*committerMock, any]{
+			b = beginnerMock[*committerMock, any]{
 				beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 					beginnerCalled = true
 					assertTrue(t, opts == nil)
@@ -75,12 +110,12 @@ func Test_Transactor(t *testing.T) {
 				},
 			}
 			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 		)
 		err := tr.WithinTx(ctx, func(ctx context.Context) error {
 			beginner := tr.TxBeginner()
 			assertTrue(t, beginner != nil)
-			assertTrue(t, b == beginner)
+			assertTrue(t, &b == beginner)
 			return nil
 		})
 		assertTrue(t, err == nil)
@@ -99,7 +134,7 @@ func Test_Transactor(t *testing.T) {
 						return nil
 					},
 				}
-				b = &beginnerMock[*committerMock, any]{
+				b = beginnerMock[*committerMock, any]{
 					beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 						beginnerCalled = true
 						assertTrue(t, opts == nil)
@@ -107,7 +142,7 @@ func Test_Transactor(t *testing.T) {
 					},
 				}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
 				tx, ok := o.Extract(ctx)
@@ -129,9 +164,9 @@ func Test_Transactor(t *testing.T) {
 						return nil
 					},
 				}
-				b  = &beginnerMock[*committerMock, any]{}
+				b  = beginnerMock[*committerMock, any]{}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			ctx = o.Inject(ctx, &c)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
@@ -155,14 +190,14 @@ func Test_Transactor(t *testing.T) {
 						return expError
 					},
 				}
-				b = &beginnerMock[*committerMock, any]{
+				b = beginnerMock[*committerMock, any]{
 					beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 						assertTrue(t, opts == nil)
 						return &c, nil
 					},
 				}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
 				tx, ok := o.Extract(ctx)
@@ -187,7 +222,7 @@ func Test_Transactor(t *testing.T) {
 						return nil
 					},
 				}
-				b = &beginnerMock[*committerMock, any]{
+				b = beginnerMock[*committerMock, any]{
 					beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 						beginCalled = true
 						assertTrue(t, opts == nil)
@@ -195,7 +230,7 @@ func Test_Transactor(t *testing.T) {
 					},
 				}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
 				tx, ok := o.Extract(ctx)
@@ -222,7 +257,7 @@ func Test_Transactor(t *testing.T) {
 						return rollbackErr
 					},
 				}
-				b = &beginnerMock[*committerMock, any]{
+				b = beginnerMock[*committerMock, any]{
 					beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 						beginCalled = true
 						assertTrue(t, opts == nil)
@@ -230,7 +265,7 @@ func Test_Transactor(t *testing.T) {
 					},
 				}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
 				tx, ok := o.Extract(ctx)
@@ -256,7 +291,7 @@ func Test_Transactor(t *testing.T) {
 						return nil
 					},
 				}
-				b = &beginnerMock[*committerMock, any]{
+				b = beginnerMock[*committerMock, any]{
 					beginFn: func(ctx context.Context, opts ...Option[any]) (*committerMock, error) {
 						beginCalled = true
 						assertTrue(t, opts == nil)
@@ -264,7 +299,7 @@ func Test_Transactor(t *testing.T) {
 					},
 				}
 				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
-				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
+				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](&b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
 				tx, ok := o.Extract(ctx)
@@ -300,7 +335,7 @@ func Test_Transactor(t *testing.T) {
 						return &c, nil
 					},
 				}
-				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
@@ -327,7 +362,7 @@ func Test_Transactor(t *testing.T) {
 						return nil, expError
 					},
 				}
-				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+				o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 				tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 			)
 			err := tr.WithinTx(ctx, func(ctx context.Context) error {
@@ -435,7 +470,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 					return &c, nil
 				},
 			}
-			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 		)
 		err := tr.WithinTx(ctx, func(ctx context.Context) error {
@@ -472,7 +507,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 					return &c, nil
 				},
 			}
-			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 		)
 
@@ -535,7 +570,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 					return &c, nil
 				},
 			}
-			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 		)
 
@@ -600,7 +635,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 					return &c, nil
 				},
 			}
-			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 		)
 
@@ -667,7 +702,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 					return &c, nil
 				},
 			}
-			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](&b)
+			o  = NewContextOperator[*beginnerMock[*committerMock, any], *committerMock](b)
 			tr = NewTransactor[*beginnerMock[*committerMock, any], *committerMock, any](b, o)
 		)
 
@@ -712,7 +747,7 @@ func Test_Transactor_recursive_call(t *testing.T) {
 	})
 }
 
-// beginnerMock was added to avoid to use external dependencies for mocking
+// beginnerMock was added to avoid to use external dependencies for mocking (pointer receiver).
 type beginnerMock[T Tx, O any] struct {
 	beginFn func(ctx context.Context, opts ...Option[O]) (T, error)
 }
@@ -721,7 +756,7 @@ func (b *beginnerMock[T, O]) BeginTx(ctx context.Context, opts ...Option[O]) (T,
 	return b.beginFn(ctx, opts...)
 }
 
-// committerMock was added to avoid to use external dependencies for mocking
+// committerMock was added to avoid to use external dependencies for mocking (pointer receiver).
 type committerMock struct {
 	commitFn   func(ctx context.Context) error
 	rollbackFn func(ctx context.Context) error
@@ -735,6 +770,28 @@ func (c *committerMock) Rollback(ctx context.Context) error {
 	return c.rollbackFn(ctx)
 }
 
+// beginnerValueMock was added to avoid to use external dependencies for mocking (value receiver).
+type beginnerValueMock[T Tx, O any] struct {
+	beginner *beginnerMock[T, O]
+}
+
+func (b beginnerValueMock[T, O]) BeginTx(ctx context.Context, opts ...Option[O]) (T, error) {
+	return b.beginner.beginFn(ctx, opts...)
+}
+
+// committerValueMock was added to avoid to use external dependencies for mocking (value receiver).
+type committerValueMock struct {
+	committer *committerMock
+}
+
+func (c committerValueMock) Commit(ctx context.Context) error {
+	return c.committer.commitFn(ctx)
+}
+
+func (c committerValueMock) Rollback(ctx context.Context) error {
+	return c.committer.commitFn(ctx)
+}
+
 // assertTrue was added to avoid to use external dependencies for mocking
 func assertTrue(t *testing.T, val bool) {
 	t.Helper()