unique ID for multi tabel? (MySQL)
Based on your story mine first idea what crosses mine mind was.

**article.** 
 article_id 
 article_name
 ...
 ...

**topic** 
 topic_id
 article_id
 topic_name
 ...
 ...


**topic_has_many_articles** (many to many relation and your connection table)
  topic_id
  article_id


You need to use correct indexing for this to scale well with large datasets.