title: How to query by reference field in MongoDB?
body: <p>I have two Mongo schemas:</p>
  <p>User:</p>
  <pre><code>{
    _id: ObjectId,
    name: String,
    country: ObjectId // Reference to schema Country
  }
  </code></pre>
  <p>Country:</p>
  <pre><code>{
    _id: ObjectId,
    name: String
  }
  </code></pre>
  <p>I want to get all users who have the country name &quot;VietNam&quot;.</p>
  <p>What kind of query (only to the User schema) can I use for this case?</p>
  <p>I want it to look like this SQL query:</p>
  <pre><code>SELECT * 
  FROM User
  JOIN Country 
  ON User.country = Country._id 
  WHERE Country.name = 'VietNam'
  </code></pre>