title: Find documents by populate match result in mongoose
body: <p>For Ecample, From this data with mongoose :</p>
  
  <p>Data <strong>Students</strong> :</p>
  
  <pre><code>[{
    "_id": ObjectId("5afbb519a7fe344ff8db67e6"),
    "name":"Jack",
    "age":20
    ...
  },{
    "_id": ObjectId("5afbb534a7fe344gf7db64e7"),
    "name":"Joni",
    "age":20
    ...
  }]
  </code></pre>
  
  <p>Data <strong>Activities</strong> :</p>
  
  <pre><code>[{
    "_id": ObjectId("5afbb554a7fe344ff8db67e9"),
    "name":"Going to Market",
    "student": ObjectId("5afbb519a7fe344ff8db67e6")
    ...
  },{
    "_id": ObjectId("5afbb784a7fe344gf7db64e2"),
    "name":"Playing with Friends",
    "student":ObjectId("5afbb534a7fe344gf7db64e7")
    ...
  }]
  </code></pre>
  
  <p>I want to find a data base on conditions in populate with regex &amp; This is what i am did :</p>
  
  <pre><code>let term = new Regex('Jack','i');        
  let result = await Activities.find({})
              .populate('student', null, { name: { $regex: term } })
              .exec();
  </code></pre>
  
  <p>But the result is always return all data of <strong>Activites</strong> (The regex query doesn't work)</p>
  
  <p>What Should I Do?</p>