diff --git a/backend/donation_tracker_canister/src/Main.mo b/backend/donation_tracker_canister/src/Main.mo
index eb24f4f..05f4625 100644
--- a/backend/donation_tracker_canister/src/Main.mo
+++ b/backend/donation_tracker_canister/src/Main.mo
@@ -46,10 +46,8 @@ actor class DonationTracker(_donation_canister_id : Text) {
     stable var donationsStable : [Donation] = [];
 
     // Map each principal to a list of donation indices (DTIs)
-    private var donationsByPrincipal = HashMap.HashMap<Principal, Buffer.Buffer<DTI>>(0, Principal.equal, Principal.hash);
-    //TODO: stable var donationsByPrincipalStable : [(Principal, Buffer.Buffer<DTI>)] = [];
-    // Alternative: use [DTI] instead of Buffer.Buffer<DTI>; less efficient but straightforward to make stable
-    // Or: stable Buffer implementation?
+    private var donationsByPrincipal = HashMap.HashMap<Principal, [DTI]>(0, Principal.equal, Principal.hash);
+    stable var donationsByPrincipalStable : [(Principal, [DTI])] = []; // Alternative: Buffer.Buffer<DTI> instead of [DTI]; more efficient but less straightforward to make stable    
 
     // Store recipients and map each recipientId to the corresponding Recipient record
     private var recipientsById = HashMap.HashMap<Types.RecipientId, Types.Recipient>(0, Text.equal, Text.hash);
@@ -222,12 +220,12 @@ actor class DonationTracker(_donation_canister_id : Text) {
 
         // Update the map for the caller's principal
         if (Principal.isAnonymous(msg.caller)) {} else {
-            let existingDonations = switch (donationsByPrincipal.get(msg.caller)) {
-                case (null) { Buffer.Buffer<DTI>(0) };
+            let existingDonations : [DTI] = switch (donationsByPrincipal.get(msg.caller)) {
+                case (null) { [] };
                 case (?ds) { ds };
             };
-            let addDonationResult = existingDonations.add(newDti);
-            donationsByPrincipal.put(msg.caller, existingDonations);
+            let addDonationResult : [DTI] = Array.append<DTI>(existingDonations, [newDti]);
+            donationsByPrincipal.put(msg.caller, addDonationResult);
         };
 
         let associatedDonations = switch (donationsByTxId.get(donationInput.paymentTransactionId)) {
@@ -273,12 +271,12 @@ actor class DonationTracker(_donation_canister_id : Text) {
                 // No donations found
                 return #Ok({ donations = [] });
             };
-            case (?dtiBuffer) {
+            case (?dtiArray) {
                 // Donations found for user
-                let dtis : [DTI] = Buffer.toArray(dtiBuffer);
+                let dtis : [DTI] = dtiArray;
                 // Iterate over dtis, get donation for each dti
                 // push to return array
-                let userDonations : Buffer.Buffer<Donation> = Buffer.Buffer<Donation>(dtiBuffer.capacity());
+                let userDonations : Buffer.Buffer<Donation> = Buffer.Buffer<Donation>(dtiArray.size());
                 for (i : Nat in dtis.keys()) {
                     userDonations.add(donations.get(dtis[i]));
                 };
@@ -587,7 +585,7 @@ actor class DonationTracker(_donation_canister_id : Text) {
     system func preupgrade() {
         // Copy the runtime state back into the stable variable before upgrade.
         donationsStable := Buffer.toArray<Donation>(donations);
-        //TODO: donationsByPrincipalStable := Iter.toArray(donationsByPrincipal.entries());
+        donationsByPrincipalStable := Iter.toArray(donationsByPrincipal.entries());
         recipientsByIdStable := Iter.toArray(recipientsById.entries());
         studentsBySchoolStable := Iter.toArray(studentsBySchool.entries());
         donationsByTxIdStable := Iter.toArray(donationsByTxId.entries());
@@ -599,8 +597,8 @@ actor class DonationTracker(_donation_canister_id : Text) {
         // After upgrade, reload the runtime state from the stable variable.
         donations := Buffer.fromArray<Donation>(donationsStable);
         donationsStable := [];
-        //TODO: donationsByPrincipal := HashMap.fromIter(Iter.fromArray(donationsByPrincipalStable), donationsByPrincipalStable.size(), Text.equal, Text.hash);
-        //TODO: donationsByPrincipalStable := [];
+        donationsByPrincipal := HashMap.fromIter(Iter.fromArray(donationsByPrincipalStable), donationsByPrincipalStable.size(), Principal.equal, Principal.hash);
+        donationsByPrincipalStable := [];
         recipientsById := HashMap.fromIter(Iter.fromArray(recipientsByIdStable), recipientsByIdStable.size(), Text.equal, Text.hash);
         recipientsByIdStable := [];
         studentsBySchool := HashMap.fromIter(Iter.fromArray(studentsBySchoolStable), studentsBySchoolStable.size(), Text.equal, Text.hash);