title: How to add conditional schema based on other field?
body: <p>I have this structure:</p>
  
  <pre><code>{
      personFullName: String,
      personMobileOS: Number // 1 = IOS, 2 = Android,
      moreDetails: Mixed
  }
  </code></pre>
  
  <p>I want to add conditional schema based on other field like this:</p>
  
  <pre><code>if (personMobileOS === 1) { // IOS
      moreDetails = { 
          iosVersion: Number, 
          loveApple: Boolean
      }
  } else if (personMobileOS === 2) { // Android
      moreDetails = {
          wantToSell: Boolean,
          wantToSellPrice: Number
          wantToSellCurrency: Number // 1 = Dollar, 2 = Euro, 3 = Pound
      }
  }
  </code></pre>
  
  <p>As you can see, the schema for "moreDetails" is conditional, it's possible to achieve this in mongoose?</p>