diff --git a/src/main/java/com/sasiedzi/event/domain/Transaction.java b/src/main/java/com/sasiedzi/event/domain/Transaction.java
index a174fb8..b642716 100644
--- a/src/main/java/com/sasiedzi/event/domain/Transaction.java
+++ b/src/main/java/com/sasiedzi/event/domain/Transaction.java
@@ -43,7 +43,7 @@ public class Transaction implements Serializable {
     @OneToMany(fetch = FetchType.LAZY, mappedBy = "transaction", cascade = CascadeType.ALL, orphanRemoval = true)
     @JsonIgnoreProperties(value = { "transaction", "event", "registration" }, allowSetters = true)
     @SortComparator(TransactionItemComparator.class)
-    private List<TransactionItem> transactionItems = Transaction.getEmptyTransactionItemSet();
+    private SortedSet<TransactionItem> transactionItems = Transaction.getEmptyTransactionItemSet();
 
     @Transient
     private UserAccount beneficiary;
@@ -152,11 +152,11 @@ public class Transaction implements Serializable {
         return this;
     }
 
-    public List<TransactionItem> getTransactionItems() {
+    public SortedSet<TransactionItem> getTransactionItems() {
         return this.transactionItems;
     }
 
-    public void setTransactionItems(List<TransactionItem> transactionItems) {
+    public void setTransactionItems(SortedSet<TransactionItem> transactionItems) {
         if (this.transactionItems != null) {
             this.transactionItems.forEach(i -> i.setTransaction(null));
         }
@@ -166,7 +166,7 @@ public class Transaction implements Serializable {
         this.transactionItems = transactionItems;
     }
 
-    public Transaction transactionItems(List<TransactionItem> transactionItems) {
+    public Transaction transactionItems(SortedSet<TransactionItem> transactionItems) {
         this.setTransactionItems(transactionItems);
         return this;
     }
@@ -213,7 +213,7 @@ public class Transaction implements Serializable {
             "}";
     }
 
-    public static List<TransactionItem> getEmptyTransactionItemSet() {
-        return new ArrayList();
+    public static SortedSet<TransactionItem> getEmptyTransactionItemSet() {
+        return new TreeSet<>(new TransactionItemComparator());
     }
 }
diff --git a/src/main/java/com/sasiedzi/event/domain/TransactionItemComparator.java b/src/main/java/com/sasiedzi/event/domain/TransactionItemComparator.java
index 7938a34..cb587a5 100644
--- a/src/main/java/com/sasiedzi/event/domain/TransactionItemComparator.java
+++ b/src/main/java/com/sasiedzi/event/domain/TransactionItemComparator.java
@@ -19,6 +19,21 @@ public class TransactionItemComparator implements Comparator<TransactionItem> {
 
     private int compareUserAccounts(UserAccount ua1, UserAccount ua2) {
         // Używamy naszego komparatora UserAccount
-        return userAccountComparator.compare(ua1, ua2);
+        int compare = userAccountComparator.compare(ua1, ua2);
+        if (compare == 0) {
+            // Porównanie id, uwzględniając null
+            if (ua1.getId() == null && ua2.getId() == null) {
+                return -1; // Jeśli oba id są null, to nie są równe, ale nie wiemy jak je uporządkować, więc zwracamy -1 (lub 1, zależnie od preferencji)
+            } else if (ua1.getId() == null) {
+                return -1; // Pusty ID jest "mniejszy" niż niepusty
+            } else if (ua2.getId() == null) {
+                return 1; // Pusty ID jest "mniejszy" niż niepusty
+            } else {
+                // Oba ID nie są null, więc porównujemy je normalnie
+                return ua1.getId().compareTo(ua2.getId());
+            }
+        } else {
+            return compare;
+        }
     }
 }
diff --git a/src/main/java/com/sasiedzi/event/service/TransactionService.java b/src/main/java/com/sasiedzi/event/service/TransactionService.java
index 3b74b5e..cb1d220 100644
--- a/src/main/java/com/sasiedzi/event/service/TransactionService.java
+++ b/src/main/java/com/sasiedzi/event/service/TransactionService.java
@@ -32,7 +32,7 @@ public class TransactionService {
     private EventService eventService;
 
     public Transaction save(Transaction transaction) {
-        List<TransactionItem> items = transaction.getTransactionItems();
+        SortedSet<TransactionItem> items = transaction.getTransactionItems();
         String beneficiaryAccountName = transaction.getBeneficiary() == null
             ? Account.Skarbiec.toString()
             : transaction.getBeneficiary().getName();