title: How to make sure send `req` after query database?
body: <p>after receiving <code>req</code> from a client, the server will query mongodb and then send <code>res</code>,</p>
  
  <pre><code>var Article = mongoose.model('articles', articleSchema);
  
  app.get('/api/topic',function(req,res){
  
      Article.find({"articleID":1},function(err,doc){
           res.json(doc)
      })
  
     res.send("the message that I don't want to send")
  }
  </code></pre>
  
  <p>but <code>Model.find(query, fields, options, callback)</code> is an async method.
  <code>res.json(doc)</code> does not work.</p>
  
  <p>How to to make sure <code>res</code> send <code>doc</code>?</p>