Mongoose TypeError: User is not a constructor
<p>I'm trying to add a subdocument to a parent schema with Mongoose and MongoDB however I'm being thrown the following error:</p>

<pre><code>TypeError: User is not a constructor
</code></pre>

<p>This is based off Mongoose's documentation on <a href="http://mongoosejs.com/docs/subdocs.html" rel="noreferrer">subdocuments</a> and I think everything is the same. How can I debug this further?</p>

<p>Router</p>

<pre><code>// Add a destination to the DB
router.post('/add', function(req, res, next) {
  let airport = req.body.destination
  let month = req.body.month
  let id = (req.user.id)

  User.findById(id , function (err, User) {
    if (err) return handleError(err)

    function addToCart (airport, month, id) {
      var user = new User ({
        destinations: [(
          airport = '',
          month = ''
        )]
      })

      dog.destinations[0].airport = airport
      dog.destinations[0].month = month
      dog.save(callback)
      res.status(200).send('added')
    }
    addToCart()
  })
  console.log(airport)
})
</code></pre>

<p>Schema</p>

<pre><code>var destinationSchema = new Schema({
  airport: String,
  month: String
})

// Define the scheme
var User = new Schema ({
  firstName: {
    type: String,
    index: true
  },
  lastName: {
    type: String,
    index: true
  },
  email: {
    type: String,
    index: true
  },
  homeAirport: {
    type: String,
    index: true
  },
  destinations: [destinationSchema]
})


User.plugin(passportLocalMongoose)

module.exports = mongoose.model('User', User)
</code></pre>