diff --git a/db/item.go b/db/item.go
index e2fff33..3dff6e4 100644
--- a/db/item.go
+++ b/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
diff --git a/db/item_bought.go b/db/item_bought.go
index d63d152..082a94c 100644
--- a/db/item_bought.go
+++ b/db/item_bought.go
@@ -15,3 +15,5 @@ type ItemBought struct {
 	MetricType  uint8
 	MetricValue uint64
 }
+
+func (i ItemBought) __internalBelogingToPayment() {}
diff --git a/db/payment.go b/db/payment.go
index 3256e4a..c05b47f 100644
--- a/db/payment.go
+++ b/db/payment.go
@@ -21,3 +21,5 @@ type Payment struct {
 	Items      []ItemBought `gorm:"constraint:OnDelete:CASCADE;"`
 	Date       time.Time
 }
+
+func (p Payment) __internalBelogingToPayment() {}