feat: follow an epic with everything under it

«Seguir todo» on a ticket with subtasks (and on each epic of the Epics
page) tells you of it and of every ticket hanging from it, at any depth,
also the ones added later: their moves, assignments, comments and plan
changes, as a follower of each, plus when a ticket is placed under it.
Guests of a private project are left out. Personal, no history (Mongo
`tree_follows`).

API: followTree / unfollowTree, Issue.followingTree, PUT/DELETE
/api/issues/{issue}/follow-tree. MCP: watch_issue with tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/follows/tree-follows.ts

@@ -0,0 +1,82 @@
+import {
+  Command,
+  CommandHandler,
+  ICommandHandler,
+  IQueryHandler,
+  Query,
+  QueryBus,
+  QueryHandler,
+} from '@nestjs/cqrs';
+import { NotFoundError } from '@task-stream/api/shared/domain';
+import { IssueReadModel } from '@task-stream/api/tracker/domain';
+import { TreeFollowsRepository } from '../ports/tree-follows.repository';
+import { GetIssuesByIdsQuery } from '../queries/get-issues-by-ids.query';
+
+/*
+ * Following a ticket with everything under it (an epic, or a ticket split
+ * into subtasks): told of its changes and of every ticket that hangs from
+ * it, at any depth, also the ones created later. Personal, no history
+ * (like pinned tickets).
+ */
+
+export class FollowTreeCommand extends Command<void> {
+  constructor(
+    public readonly userId: string,
+    public readonly issueId: string,
+    /** False: stop following it. */
+    public readonly on: boolean,
+  ) {
+    super();
+  }
+}
+
+export class IsFollowingTreeQuery extends Query<boolean> {
+  constructor(
+    public readonly userId: string,
+    public readonly issueId: string,
+  ) {
+    super();
+  }
+}
+
+@CommandHandler(FollowTreeCommand)
+export class FollowTreeCommandHandler
+  implements ICommandHandler<FollowTreeCommand>
+{
+  constructor(
+    private readonly follows: TreeFollowsRepository,
+    private readonly queryBus: QueryBus,
+  ) {}
+
+  async execute(command: FollowTreeCommand): Promise<void> {
+    const { userId, issueId } = command;
+    if (!command.on) return this.follows.unfollow(userId, issueId);
+    const [issue]: IssueReadModel[] = await this.queryBus.execute(
+      new GetIssuesByIdsQuery(userId, [issueId]),
+    );
+    // A ticket I cannot read does not exist, as far as I know.
+    if (!issue || issue.trashedAt) throw new NotFoundError('Issue', issueId);
+    await this.follows.follow({
+      userId,
+      issueId,
+      projectId: issue.projectId,
+      since: new Date().toISOString(),
+    });
+  }
+}
+
+@QueryHandler(IsFollowingTreeQuery)
+export class IsFollowingTreeQueryHandler
+  implements IQueryHandler<IsFollowingTreeQuery, boolean>
+{
+  constructor(private readonly follows: TreeFollowsRepository) {}
+
+  execute(query: IsFollowingTreeQuery): Promise<boolean> {
+    return this.follows.isFollowing(query.userId, query.issueId);
+  }
+}
+
+export const TREE_FOLLOWS = [
+  FollowTreeCommandHandler,
+  IsFollowingTreeQueryHandler,
+];

libs/api/tracker/application/src/lib/notifications/notifications.event-handler.ts

@@ -3,6 +3,7 @@ import {
   CommentAddedEvent,
   IssueAssignedEvent,
   IssueCreatedEvent,
+  IssueParentChangedEvent,
   IssuePriority,
   IssueSprintChangedEvent,
   IssueStatusChangedEvent,
@@ -24,6 +25,7 @@ import { FindCommentsRepository } from '../ports/find-comments.repository';
 import { FindIssuesRepository } from '../ports/find-issues.repository';
 import { FindProjectsRepository } from '../ports/find-projects.repository';
 import { FindSprintsRepository } from '../ports/find-sprints.repository';
+import { TreeFollowsRepository } from '../ports/tree-follows.repository';
 import { UserDirectory } from '../ports/user-directory';
 import { NotificationDraft, Notifier } from './notifier';
 
@@ -61,10 +63,12 @@ const excerpt = (text: string, max = 120) => {
  */
 @EventsHandler(
   IssueCreatedEvent,
+  IssueParentChangedEvent,
   IssueAssignedEvent,
   IssueStatusChangedEvent,
   IssueUpdatedEvent,
   IssueSprintChangedEvent,
+  IssueParentChangedEvent,
   CommentAddedEvent,
   ProjectMemberAddedEvent,
 )
@@ -76,6 +80,7 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
     private readonly users: UserDirectory,
     private readonly findComments: FindCommentsRepository,
     private readonly findSprints: FindSprintsRepository,
+    private readonly treeFollows: TreeFollowsRepository,
   ) {}
 
   async handle(event: TrackerEvent): Promise<void> {
@@ -118,9 +123,23 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
     const assigneeId =
       'assigneeId' in issue ? issue.assigneeId : (issue.assignee?.id ?? null);
     const ticket = `${issue.key}: ${issue.title}`;
-    const watcherIds =
-      'watchers' in issue ? issue.watchers.map((w) => w.id) : [];
     const memberIds = new Set(project.members.map((m) => m.user.id));
+    // Who follows it with everything under it, or any ticket above it.
+    // Placed under another ticket: its new place counts (the read model may
+    // not have it yet).
+    const parentId =
+      event instanceof IssueParentChangedEvent
+        ? event.parentId
+        : 'parent' in issue
+          ? (issue.parent?.id ?? null)
+          : null;
+    const tree = await this.treeFollowersOf(issue.id, parentId, project);
+    const watcherIds = [
+      ...new Set([
+        ...('watchers' in issue ? issue.watchers.map((w) => w.id) : []),
+        ...tree.followers,
+      ]),
+    ];
 
     const drafts: NotificationDraft[] = [];
     // A proposal from the public form: whoever runs the project should know.
@@ -142,6 +161,16 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
         });
       }
     }
+    // A ticket new under a followed one (created there or moved in): the
+    // only way to hear of it.
+    if (event instanceof IssueParentChangedEvent && event.parentId) {
+      for (const userId of tree.followers)
+        drafts.push({
+          userId,
+          kind: NotificationKind.WATCHED,
+          summary: `${actor} añadió ${ticket} a ${event.parentKey ?? tree.parentKey}`,
+        });
+    }
     if (event instanceof IssueCreatedEvent && assigneeId) {
       drafts.push({
         userId: assigneeId,
@@ -271,6 +300,40 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
     );
   }
 
+  /**
+   * Those following the ticket with its subtree, or any ticket above it
+   * (an epic, a parent), who can read it all: members that are not guests
+   * limited to their own tickets. Also the key of its parent.
+   */
+  private async treeFollowersOf(
+    issueId: string,
+    parentId: string | null,
+    project: {
+      isPublic: boolean;
+      members: { user: { id: string }; role: ProjectRole }[];
+    },
+  ): Promise<{ followers: string[]; parentKey: string | null }> {
+    const ids = [issueId];
+    let parentKey: string | null = null;
+    let next = parentId;
+    // A few levels: epic → story → subtask is as deep as trees go.
+    for (let depth = 0; next && depth < 5 && !ids.includes(next); depth++) {
+      ids.push(next);
+      const parent = await this.findIssues.findById(next);
+      parentKey ??= parent?.key ?? null;
+      next = parent?.parent?.id ?? null;
+    }
+    const readers = new Set(
+      project.members
+        .filter((m) => project.isPublic || m.role !== ProjectRole.GUEST)
+        .map((m) => m.user.id),
+    );
+    const followers = (await this.treeFollows.followersOf(ids)).filter((id) =>
+      readers.has(id),
+    );
+    return { followers, parentKey };
+  }
+
   /**
    * What a follower should hear of a change of priority, due date or
    * sprint; null for the other edits (title, labels…), too small to tell.

libs/api/tracker/application/src/lib/notifications/notifications.spec.ts

@@ -5,6 +5,7 @@ import {
   IssuePriority,
   IssueSprintChangedEvent,
   IssueStatusChangedEvent,
+  IssueParentChangedEvent,
   IssueUpdatedEvent,
   NotificationKind,
   NotificationReadModel,
@@ -23,6 +24,7 @@ import { FindProjectsRepository } from '../ports/find-projects.repository';
 import { NotificationMailer } from '../ports/notification-mailer';
 import { NotificationsRepository } from '../ports/notifications.repository';
 import { NotificationsEventHandler } from './notifications.event-handler';
+import { TreeFollowsRepository } from '../ports/tree-follows.repository';
 import { Notifier } from './notifier';
 import {
   PushPayload,
@@ -62,7 +64,12 @@ const at = <T extends TrackerEvent>(event: T, version = 2): T =>
     occurredAt: '2026-10-08T10:00:00.000Z',
   });
 
-function setup(mailEnabled = false, pushEnabled = false) {
+function setup(
+  mailEnabled = false,
+  pushEnabled = false,
+  /** Ticket id → who follows it with everything under it. */
+  follows: Record<string, string[]> = {},
+) {
   const repo = new InMemoryNotifications();
   const published: NotificationReadModel[] = [];
   const mailed: string[] = [];
@@ -74,7 +81,9 @@ function setup(mailEnabled = false, pushEnabled = false) {
     .add('ana')
     .add('bob')
     .add('qa')
-    .add('pm');
+    .add('pm')
+    .add('lia')
+    .add('gus');
   const issue = {
     id: 'i-1',
     key: 'WEB-1',
@@ -85,7 +94,10 @@ function setup(mailEnabled = false, pushEnabled = false) {
       { id: 'pm', name: 'PM' },
       { id: 'bob', name: 'Bob' },
     ],
+    parent: { id: 'e-1', key: 'WEB-10', title: 'Checkout' },
   };
+  /** The epic it hangs from. */
+  const epic = { id: 'e-1', key: 'WEB-10', title: 'Checkout', parent: null };
   const notifier = new Notifier(
     repo,
     {
@@ -118,15 +130,23 @@ function setup(mailEnabled = false, pushEnabled = false) {
   );
   const handler = new NotificationsEventHandler(
     notifier,
-    { findById: async () => issue } as unknown as FindIssuesRepository,
+    {
+      findById: async (id: string) => (id === 'e-1' ? epic : issue),
+    } as unknown as FindIssuesRepository,
     {
       findById: async () => ({
         id: 'p-1',
         key: 'WEB',
         name: 'Web',
-        members: ['ana', 'bob', 'qa', 'pm'].map((id) => ({
+        isPublic: false,
+        members: ['ana', 'bob', 'qa', 'pm', 'lia', 'gus'].map((id) => ({
           user: { id, name: id },
-          role: id === 'ana' ? ProjectRole.OWNER : ProjectRole.MEMBER,
+          role:
+            id === 'ana'
+              ? ProjectRole.OWNER
+              : id === 'gus'
+                ? ProjectRole.GUEST
+                : ProjectRole.MEMBER,
         })),
       }),
     } as unknown as FindProjectsRepository,
@@ -142,11 +162,49 @@ function setup(mailEnabled = false, pushEnabled = false) {
       findById: async (id: string) =>
         id === 's-14' ? { id, name: 'Sprint 14' } : null,
     } as unknown as FindSprintsRepository,
+    {
+      followersOf: async (ids: string[]) => [
+        ...new Set(ids.flatMap((id) => follows[id] ?? [])),
+      ],
+    } as unknown as TreeFollowsRepository,
   );
   return { repo, published, mailed, pushed, devices, forgotten, handler };
 }
 
 describe('notifications', () => {
+  it('tells whoever follows the epic of what happens to its tickets, not guests', async () => {
+    const { handler, repo } = setup(false, false, { 'e-1': ['lia', 'gus'] });
+    await handler.handle(
+      at(
+        new IssueStatusChangedEvent(
+          'ana',
+          'i-1',
+          'p-1',
+          'TODO',
+          'IN_PROGRESS',
+          null,
+        ),
+      ),
+    );
+    const lia = repo.all.filter((n) => n.userId === 'lia');
+    expect(lia.map((n) => n.kind)).toEqual([NotificationKind.WATCHED]);
+    expect(lia[0].summary).toContain('WEB-1');
+    // A guest only sees their own tickets: not told of the epic's.
+    expect(repo.all.some((n) => n.userId === 'gus')).toBe(false);
+  });
+
+  it('tells whoever follows the epic of a ticket added to it', async () => {
+    const { handler, repo } = setup(false, false, { 'e-1': ['lia'] });
+    await handler.handle(
+      at(
+        new IssueParentChangedEvent('ana', 'i-1', 'p-1', 'e-1', 'WEB-10', null),
+      ),
+    );
+    expect(repo.all.map((n) => [n.userId, n.summary])).toEqual([
+      ['lia', expect.stringContaining('añadió WEB-1: Login roto a WEB-10')],
+    ]);
+  });
+
   it('tells whoever a ticket is assigned to, not whoever assigned it', async () => {
     const { handler, repo, published } = setup();
     await handler.handle(

libs/api/tracker/application/src/lib/ports/tree-follows.repository.ts

@@ -0,0 +1,16 @@
+/** Who follows a ticket with everything under it: personal, no history. */
+export interface TreeFollow {
+  userId: string;
+  issueId: string;
+  projectId: string;
+  since: string;
+}
+
+export abstract class TreeFollowsRepository {
+  /** Following it again keeps the first date. */
+  abstract follow(follow: TreeFollow): Promise<void>;
+  abstract unfollow(userId: string, issueId: string): Promise<void>;
+  abstract isFollowing(userId: string, issueId: string): Promise<boolean>;
+  /** Everyone following any of these tickets (each once). */
+  abstract followersOf(issueIds: string[]): Promise<string[]>;
+}

libs/api/tracker/application/src/lib/tracker-application.module.ts

@@ -37,6 +37,8 @@ import { SAVED_FILTERS } from './saved-filters/saved-filters';
 import { MY_VIEWS } from './saved-filters/my-views';
 import { PINS } from './saved-filters/pins';
 import { ALERT_HANDLERS } from './alerts/alerts';
+import { TREE_FOLLOWS } from './follows/tree-follows';
+import { TreeFollowsService } from './tree-follows.service';
 import { AlertsService } from './alerts.service';
 import { PinsService } from './pins.service';
 import { SendDueRemindersCommandHandler } from './notifications/reminders';
@@ -84,6 +86,7 @@ const SERVICES = [
   SavedFiltersService,
   PinsService,
   AlertsService,
+  TreeFollowsService,
   SprintsService,
   GitService,
   WebhooksService,
@@ -116,6 +119,7 @@ const SERVICES = [
     ...MY_VIEWS,
     ...PINS,
     ...ALERT_HANDLERS,
+    ...TREE_FOLLOWS,
     ...GIT_HANDLERS,
     ...WEBHOOK_HANDLERS,
     ...TRANSFER_HANDLERS,

libs/api/tracker/application/src/lib/tree-follows.service.ts

@@ -0,0 +1,24 @@
+import { CommandRunner } from '@task-stream/api/shared/application';
+import { Injectable } from '@nestjs/common';
+import { QueryBus } from '@nestjs/cqrs';
+import {
+  FollowTreeCommand,
+  IsFollowingTreeQuery,
+} from './follows/tree-follows';
+
+/** Following a ticket with everything under it (an epic). */
+@Injectable()
+export class TreeFollowsService {
+  constructor(
+    private readonly commands: CommandRunner,
+    private readonly queryBus: QueryBus,
+  ) {}
+
+  follow(userId: string, issueId: string, on: boolean): Promise<void> {
+    return this.commands.execute(new FollowTreeCommand(userId, issueId, on));
+  }
+
+  isFollowing(userId: string, issueId: string): Promise<boolean> {
+    return this.queryBus.execute(new IsFollowingTreeQuery(userId, issueId));
+  }
+}