diff --git a/db/transfer.go b/db/transfer.go
index d5d79d5..bb039a4 100644
--- a/db/transfer.go
+++ b/db/transfer.go
@@ -1,6 +1,7 @@
 package db
 
 import (
+	"errors"
 	"time"
 
 	"gorm.io/gorm"
@@ -17,3 +18,144 @@ type Transfer struct {
 	UserID     uint
 	User       *User
 }
+
+// Implements db.UserIdentifiable:1
+func (t Transfer) GetID() uint {
+	return t.ID
+}
+
+// Implements db.UserIdentifiable:2
+func (t Transfer) GetUserID() uint {
+	return t.UserID
+}
+
+// Implements db.UserIdentifiable:3
+func (t *Transfer) SetUserID(id uint) {
+	t.UserID = id
+}
+
+var (
+	ERROR_TRANSFER_FROMCARD_INVALID_USERID       = errors.New("Transfer's UserID and FromCard's UserID are not equal")
+	ERROR_TRANSFER_FROMCARD_INSUFFICIENT_BALANCE = errors.New("FromCard's Balance is lower than Transfer's Value")
+	ERROR_TRANSFER_TOCARD_INVALID_USERID         = errors.New("Transfer's UserID and ToCard's UserID are not equal")
+)
+
+func (t *Transfer) BeforeCreate(tx *gorm.DB) error {
+	fromCard := &Card{}
+	if err := tx.Find(fromCard, t.FromCardID).Error; err != nil {
+		return err
+	}
+	if fromCard.UserID != t.UserID {
+		return ERROR_TRANSFER_FROMCARD_INVALID_USERID
+	}
+	if fromCard.Balance < t.Value {
+		return ERROR_TRANSFER_FROMCARD_INSUFFICIENT_BALANCE
+	}
+	fromCard.Balance -= t.Value
+	if err := tx.Save(fromCard).Error; err != nil {
+		return err
+	}
+	//
+	toCard := &Card{}
+	if err := tx.Find(toCard, t.ToCardID).Error; err != nil {
+		return err
+	}
+	if toCard.UserID != t.UserID {
+		return ERROR_TRANSFER_TOCARD_INVALID_USERID
+	}
+	toCard.Balance += t.Value
+	if err := tx.Save(toCard).Error; err != nil {
+		return err
+	}
+	return nil
+}
+
+var (
+	ERROR_TRANSFER_FROMCARD_IDZERO             = errors.New("Transfer's FromCardID is zero")
+	ERROR_TRANSFER_TOCARD_IDZERO               = errors.New("Transfer's ToCardID is zero")
+	ERROR_TRANSFER_TOCARD_INSUFFICIENT_BALANCE = errors.New("Transfer's ToCard's Balance is lower than value of transfer")
+)
+
+func (t *Transfer) BeforeUpdate(tx *gorm.DB) error {
+	var original Transfer
+	if err := tx.Find(&original, t.ID).Error; err != nil {
+		return err
+	}
+	if original.FromCardID == 0 {
+		return ERROR_TRANSFER_FROMCARD_IDZERO
+	}
+	var origFromCard Card
+	if err := tx.Find(&origFromCard, original.FromCardID).Error; err != nil {
+		return err
+	}
+	origFromCard.Balance += original.Value
+	if err := tx.Save(origFromCard).Error; err != nil {
+		return err
+	}
+
+	if original.ToCardID == 0 {
+		return ERROR_TRANSFER_FROMCARD_IDZERO
+	}
+	var origToCard Card
+	if err := tx.Find(&origToCard, original.ToCardID).Error; err != nil {
+		return err
+	}
+	if origToCard.Balance < original.Value {
+		return ERROR_TRANSFER_TOCARD_INSUFFICIENT_BALANCE
+	}
+	origToCard.Balance -= original.Value
+	if err := tx.Save(origToCard).Error; err != nil {
+		return err
+	}
+	//
+	fromCard := &Card{}
+	if err := tx.Find(fromCard, t.FromCardID).Error; err != nil {
+		return err
+	}
+	if fromCard.UserID != t.UserID {
+		return ERROR_TRANSFER_FROMCARD_INVALID_USERID
+	}
+	if fromCard.Balance < t.Value {
+		return ERROR_TRANSFER_FROMCARD_INSUFFICIENT_BALANCE
+	}
+	fromCard.Balance -= t.Value
+	if err := tx.Save(fromCard).Error; err != nil {
+		return err
+	}
+	//
+	toCard := &Card{}
+	if err := tx.Find(toCard, t.ToCardID).Error; err != nil {
+		return err
+	}
+	if toCard.UserID != t.UserID {
+		return ERROR_TRANSFER_TOCARD_INVALID_USERID
+	}
+	toCard.Balance += t.Value
+	if err := tx.Save(toCard).Error; err != nil {
+		return err
+	}
+	return nil
+}
+
+func (t *Transfer) AfterDelete(tx *gorm.DB) error {
+	fromCard := &Card{}
+	if err := tx.Find(fromCard, t.FromCardID).Error; err != nil {
+		return err
+	}
+	fromCard.Balance += t.Value
+	if err := tx.Save(fromCard).Error; err != nil {
+		return err
+	}
+	toCard := &Card{}
+	if err := tx.Find(toCard, t.ToCardID).Error; err != nil {
+		return err
+	}
+	if toCard.Balance < t.Value {
+		return ERROR_TRANSFER_TOCARD_INSUFFICIENT_BALANCE
+	}
+	toCard.Balance -= t.Value
+	if err := tx.Save(toCard).Error; err != nil {
+		return err
+	}
+	return nil
+}