fix: postgresql requires specific syntax for upsert

components/backend/internal/service/hackathon_service.go

@@ -276,15 +276,15 @@ func (s *HackathonService) Join(
 	}
 
 	// Check if user already exists in hackathon (approved or waitlisted)
-	_, err = s.dbClient.Participant.Query().Where(
+	participant, err := s.dbClient.Participant.Query().Where(
 		entparticipant.HackathonIDEQ(id),
 		entparticipant.UserID(user.ID),
 	).Only(ctx)
+	isMember := false
 	if err == nil {
-		// Already a participant - return success with existing hackathon ID
-		return &msgs.JoinResponse{HackathonId: h.ID.String()}, nil
-	}
-	if !ent.IsNotFound(err) {
+		// Already a participant - we don't need to insert, just potentially upsert answers
+		isMember = true
+	} else if !ent.IsNotFound(err) {
 		slog.Error("check existing participant", "err", err)
 
 		return nil, status.Error(codes.Internal, "couldn't check participant status")
@@ -302,15 +302,17 @@ func (s *HackathonService) Join(
 		return nil, status.Error(codes.Internal, "couldn't start transaction")
 	}
 
-	participant, err := tx.Participant.Create().
-		SetHackathonID(id).
-		SetUserID(user.ID).
-		SetIsWaiting(true).
-		Save(ctx)
-	if err != nil {
-		_ = tx.Rollback()
-		slog.Error("create participant", "err", err)
-		return nil, status.Errorf(codes.Internal, "couldn't join hackathon")
+	if !isMember {
+		participant, err = tx.Participant.Create().
+			SetHackathonID(id).
+			SetUserID(user.ID).
+			SetIsWaiting(true).
+			Save(ctx)
+		if err != nil {
+			_ = tx.Rollback()
+			slog.Error("create participant", "err", err)
+			return nil, status.Errorf(codes.Internal, "couldn't join hackathon")
+		}
 	}
 
 	// Upsert answers linked to the new participant
@@ -320,7 +322,7 @@ func (s *HackathonService) Join(
 			SetQuestionID(qID).
 			SetUserID(participant.UserID).
 			SetValue(protoAnswerValueToDB(a)).
-			OnConflict().
+			OnConflictColumns(entanswer.FieldQuestionID, entanswer.FieldUserID).
 			UpdateNewValues().
 			Exec(ctx)
 		if err != nil {
@@ -1542,7 +1544,7 @@ func (s *HackathonService) SubmitAnswers(
 			SetQuestionID(qID).
 			SetUserID(participant.UserID).
 			SetValue(protoAnswerValueToDB(a)).
-			OnConflict().
+			OnConflictColumns(entanswer.FieldQuestionID, entanswer.FieldUserID).
 			UpdateNewValues().
 			Exec(ctx)
 		if err != nil {

components/backend/internal/service/hackathon_service_test.go

@@ -363,6 +363,84 @@ var _ = Describe("HackathonService", func() {
 			Expect(participant.IsWaiting).To(BeTrue())
 		})
 
+		It("upserts answers when user joins twice with different answers", func() {
+			// Create a question first
+			token := testutils.CreateTestJWTToken(testAdmin)
+			adminCtx := metadata.NewOutgoingContext(
+				context.Background(),
+				metadata.Pairs("authorization", "Bearer "+token),
+			)
+			qResp, err := client.CreateQuestion(adminCtx, &msgs.CreateQuestionRequest{
+				HackathonId: createdHackathonID,
+				Key:         "company",
+				Label:       "Company",
+				Type:        entities.QuestionType_QUESTION_TYPE_TEXT,
+				Mandatory:   true,
+				Order:       1,
+			})
+			Expect(err).NotTo(HaveOccurred())
+			questionID := qResp.GetQuestionId()
+
+			// Create a user
+			joinUserKeycloakID := "join-upsert-user"
+			joinUser, err := dbClient.User.Create().
+				SetKeycloakID(joinUserKeycloakID).
+				SetUsername("join-upsert-username").
+				Save(context.Background())
+			Expect(err).NotTo(HaveOccurred())
+
+			// Join first time with answer "Acme Corp"
+			joinToken := testutils.CreateTestJWTToken(joinUserKeycloakID)
+			joinCtx := metadata.NewOutgoingContext(
+				context.Background(),
+				metadata.Pairs("authorization", "Bearer "+joinToken),
+			)
+			_, err = client.Join(joinCtx, &msgs.JoinRequest{
+				HackathonId: createdHackathonID,
+				Answers: []*entities.Answer{
+					{
+						QuestionId: questionID,
+						Value:      &entities.Answer_TextValue{TextValue: "Acme Corp"},
+					},
+				},
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			// Verify first answer
+			answers, err := dbClient.Answer.Query().
+				Where(
+					entanswer.QuestionIDEQ(uuid.MustParse(questionID)),
+					entanswer.UserID(joinUser.ID),
+				).All(context.Background())
+			Expect(err).NotTo(HaveOccurred())
+			Expect(answers).To(HaveLen(1))
+			Expect(answers[0].Value).To(Equal("Acme Corp"))
+
+			// Join again with different answer "Globex Inc"
+			_, err = client.Join(joinCtx, &msgs.JoinRequest{
+				HackathonId: createdHackathonID,
+				Answers: []*entities.Answer{
+					{
+						QuestionId: questionID,
+						Value:      &entities.Answer_TextValue{TextValue: "Globex Inc"},
+					},
+				},
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			// Verify answer was upserted (not duplicated)
+			answers, err = dbClient.Answer.Query().
+				Where(
+					entanswer.QuestionIDEQ(uuid.MustParse(questionID)),
+					entanswer.UserID(joinUser.ID),
+				).All(context.Background())
+			Expect(err).NotTo(HaveOccurred())
+			Expect(answers).To(HaveLen(1), "should have exactly one answer, not duplicate")
+			Expect(
+				answers[0].Value,
+			).To(Equal("Globex Inc"), "answer should be updated to new value")
+		})
+
 		It("returns NOT_FOUND for invalid hackathon ID", func() {
 			token := testutils.CreateTestJWTToken(testAdmin)
 			ctx := metadata.NewOutgoingContext(