title: Mongoose: how to use aggregate and find together
body: <p>How can I use <a href="http://mongoosejs.com/docs/api.html#index_Mongoose-Aggregate" rel="noreferrer">aggregate</a> and <code>find</code> together in Mongoose?<br>
  i.e I have the following schema:</p>
  
  <pre><code>const schema = new Mongoose.Schema({
    created: { type: Date, default: Date.now() },
    name: { type: String, default: 'development' }
    followers: [{ type: Mongoose.Schema.ObjectId, ref: 'Users'}]
  ...
  })
  
  export default Mongoose.model('Locations', schema)
  </code></pre>
  
  <p>How can I query the users with only the fields <code>name</code> and <code>followers_count</code>.<br>
  <code>followers_count</code>:  the length of <code>followers</code> . </p>
  
  <p>There, I know we can use <a href="http://mongoosejs.com/docs/api.html#query_Query-select" rel="noreferrer">select</a> to get only the field <code>name</code>.<br>
  How can we get the count of <code>followers</code>?</p>