title: Check if ID exists in a collection with mongoose
body: <p>For instance, I have a collection <code>User</code>:</p>
  
  <pre><code>var mongoose = require('mongoose');
  
  var UserSchema = new mongoose.Schema({
      email: String,
      googleId: String,
      facebookId: String,
      displayName: String,
      active: Boolean
  });
  
  module.exports = mongoose.model('User', UserSchema);
  </code></pre>
  
  <p>And then I have an ID:</p>
  
  <pre><code>var userID = "some-user-id"
  </code></pre>
  
  <p>What is the right way to just check if this id exists in the <code>User</code> collection. I don't need it to read the file or return it, I just need the <code>true</code> or <code>false</code> value.</p>
  
  <p>Here is one way to achieve it:</p>
  
  <pre><code>User.findOne({
       _id: userID
  }, function (err, existingUser) {
  </code></pre>
  
  <p><strong>But is there faster and more efficient way?</strong></p>