title: Restrict mongoose field values
body: <p>I'm trying to create:</p>
  
  <pre><code>var mongoose = require('mongoose');
  
  var FeelingSchema = new mongoose.Schema({
      userId: String,
      feelingDate: Date,
      feelingTimeOfDay: String,
      feelingValue: String
    }
  );
  </code></pre>
  
  <p>How do I restrict the value of the field feelingValue to a limited set, say ['happy', 'angry', 'shocked']</p>
  
  <p>I'm using version 3.8.23 of mongoose</p>