Add index

src/models/post-watching.ts

@@ -2,6 +2,7 @@ import * as mongo from 'mongodb';
 import db from '../db/mongodb';
 
 const PostWatching = db.get<IPostWatching>('postWatching');
+PostWatching.createIndex(['userId', 'postId'], { unique: true });
 export default PostWatching;
 
 export interface IPostWatching {