title: How to add property to document during developing, but don't save to database?
body: <pre><code>VK_Accounts_schema.pre('init', function(doc) {
    doc.vk = new VK(doc.access_token);
    return doc;
  })
  
  VK_Accounts_schema.methods.get_API = function(cb) {
    console.log(this); // i can see property vk
    this.vk(...) // ERROR undefined!!!
  }
  </code></pre>
  
  <p>I necessary need to add the vk exactly into the document, i do not need to save it to the database.</p>