title: Move subdocument to another document
body: <p>I'm modelling a mongodb database that will have categories and articles.</p>
  
  <p>The most common use will be listing articles through the categories, so I plan to make the articles as subdocuments of categories:</p>
  
  <p>//db.categories.find()</p>
  
  <pre><code>[{
    "id": 01,
    "name": "Some category",
    "articles: [{
      "article_id": 01,
      "title": "",
      "content": ""
    }]
  }]
  </code></pre>
  
  <p>Sometimes, I might be moving articles from one category to another.</p>
  
  <p>In a relational database, I should just update a foreign key, but in the above case, how can I move an article to another category?</p>
  
  <p>Some details:</p>
  
  <ul>
  <li>Preferable, the internal <code>ObjectId</code> should remain the same. But it's not mandatory.</li>
  <li>Article might also contain subdocuments, if so, they must come along when moving.</li>
  </ul>