Add Paymenr Item and ItemBought as entities in PaymentGroup for db interface resolution

db/item.go

@@ -27,6 +27,8 @@ type Item struct {
 	User   *User
 }
 
+func (i Item) __internalBelogingToPayment() {}
+
 // Implements db.UserIdentifiable:1
 func (i Item) GetID() uint {
 	return i.ID

db/item_bought.go

@@ -15,3 +15,5 @@ type ItemBought struct {
 	MetricType  uint8
 	MetricValue uint64
 }
+
+func (i ItemBought) __internalBelogingToPayment() {}

db/payment.go

@@ -21,3 +21,5 @@ type Payment struct {
 	Items      []ItemBought `gorm:"constraint:OnDelete:CASCADE;"`
 	Date       time.Time
 }
+
+func (p Payment) __internalBelogingToPayment() {}