timers should not be used on issues with multiple assignees

packages/backend/src/db/queries/issues.ts

@@ -182,3 +182,11 @@ export async function getIssuesWithUsersByProject(projectId: number): Promise<Is
         Assignees: assigneesByIssue.get(row.Issue.id) || [],
     }));
 }
+
+export async function getIssueAssigneeCount(issueId: number): Promise<number> {
+    const [result] = await db
+        .select({ count: sql<number>`COUNT(*)` })
+        .from(IssueAssignee)
+        .where(eq(IssueAssignee.issueId, issueId));
+    return result?.count ?? 0;
+}

packages/backend/src/routes/timer/toggle.ts

@@ -5,7 +5,12 @@ import {
     TimerToggleRequestSchema,
 } from "@sprint/shared";
 import type { AuthedRequest } from "../../auth/middleware";
-import { appendTimestamp, createTimedSession, getActiveTimedSession } from "../../db/queries";
+import {
+    appendTimestamp,
+    createTimedSession,
+    getActiveTimedSession,
+    getIssueAssigneeCount,
+} from "../../db/queries";
 import { parseJsonBody } from "../../validation";
 
 export default async function timerToggle(req: AuthedRequest) {
@@ -14,6 +19,14 @@ export default async function timerToggle(req: AuthedRequest) {
 
     const { issueId } = parsed.data;
 
+    const assigneeCount = await getIssueAssigneeCount(issueId);
+    if (assigneeCount > 1) {
+        return Response.json(
+            { error: "Timers cannot be used on issues with multiple assignees", code: "MULTIPLE_ASSIGNEES" },
+            { status: 400 },
+        );
+    }
+
     const activeSession = await getActiveTimedSession(req.userId, issueId);
 
     if (!activeSession) {