feat(social): prevent users from following themselves (BA-053)

BackendAcademy/src/social/dto/follow.dto.ts

@@ -1,11 +1,19 @@
+import { ApiProperty } from '@nestjs/swagger';
 import { IsNotEmpty, IsString } from 'class-validator';
 
 export class FollowDto {
+  @ApiProperty({ example: 'learner-1', description: 'The user initiating the follow request' })
   @IsString()
   @IsNotEmpty()
   followerId: string;
 
+  @ApiProperty({ example: 'tutor-1', description: 'The target user to follow (must not be self)' })
   @IsString()
   @IsNotEmpty()
   followeeId: string;
 }
+
+export interface SelfFollowError {
+  code: 'CANNOT_FOLLOW_SELF';
+  message: string;
+}

BackendAcademy/src/social/follow.service.spec.ts

@@ -1,4 +1,5 @@
 import { BadRequestException } from '@nestjs/common';
+import { FollowDto } from './dto/follow.dto';
 import { FollowService } from './follow.service';
 import { DEFAULT_RANK_WEIGHT, FOLLOW_RANK_WEIGHT } from './social.types';
 
@@ -152,4 +153,32 @@ describe('FollowService', () => {
       expect(service.getFeed('nobody')).toEqual([]);
     });
   });
+
+  // ── BA-053: Prevent users from following themselves ──────────────────────
+
+  describe('BA-053 — Prevent users from following themselves', () => {
+    it('rejects self-follow with structured CANNOT_FOLLOW_SELF error code', () => {
+      let caught: BadRequestException | undefined;
+      try {
+        service.follow('learner-self', 'learner-self');
+      } catch (err) {
+        caught = err as BadRequestException;
+      }
+
+      expect(caught).toBeInstanceOf(BadRequestException);
+      expect(caught!.getStatus()).toBe(400);
+      const res = caught!.getResponse() as Record<string, unknown>;
+      expect(res.code).toBe('CANNOT_FOLLOW_SELF');
+      expect(res.message).toBe('An account cannot follow itself');
+    });
+
+    it('rejects self-follow passed via DTO object', () => {
+      const dto: FollowDto = { followerId: 'user-99', followeeId: 'user-99' };
+      expect(() => service.follow(dto)).toThrow(BadRequestException);
+    });
+
+    it('rejects self-follow with trailing or leading whitespace', () => {
+      expect(() => service.follow('user-trim ', ' user-trim')).toThrow(BadRequestException);
+    });
+  });
 });

BackendAcademy/src/social/follow.service.ts

@@ -1,4 +1,5 @@
 import { BadRequestException, Injectable } from '@nestjs/common';
+import { FollowDto, SelfFollowError } from './dto/follow.dto';
 import {
   DEFAULT_RANK_WEIGHT,
   FeedItem,
@@ -35,9 +36,22 @@ export class FollowService {
     return `${followerId}|${followeeId}`;
   }
 
-  follow(followerId: string, followeeId: string): FollowEdge {
-    if (followerId === followeeId) {
-      throw new BadRequestException('An account cannot follow itself');
+  follow(followerId: string, followeeId: string): FollowEdge;
+  follow(dto: FollowDto): FollowEdge;
+  follow(followerOrDto: string | FollowDto, maybeFolloweeId?: string): FollowEdge {
+    const followerId = typeof followerOrDto === 'object' ? followerOrDto.followerId : followerOrDto;
+    const followeeId = typeof followerOrDto === 'object' ? followerOrDto.followeeId : maybeFolloweeId!;
+
+    if (!followerId || !followeeId) {
+      throw new BadRequestException('Follower and followee IDs must be provided');
+    }
+
+    if (followerId === followeeId || followerId.trim() === followeeId.trim()) {
+      const errorPayload: SelfFollowError = {
+        code: 'CANNOT_FOLLOW_SELF',
+        message: 'An account cannot follow itself',
+      };
+      throw new BadRequestException(errorPayload);
     }
 
     const key = FollowService.edgeKey(followerId, followeeId);