title: Mongoose Model.estimatedDocumentCount() TypeError
body: <p>I keep receiving the following error when trying to use estimatedDocumentCount function:
  "TypeError: URLModel.estimatedDocumentCount is not a function"</p>
  
  <p>Here is my code: </p>
  
  <pre><code>var mongo = require('mongodb'); //version: "^2.2.5"
  var mongoose = require('mongoose'); //version "^4.5.7"
  
  mongoose.connect(process.env.MONGO_URI, function(err, db) {
   if (err) {console.log(err);}
  });
  
  var URLSchema = mongoose.Schema({
    original_url: {type: String, required: true, unique: true},
    short_url: {type: Number, required: true, unique: true}
  });
  var URLModel = mongoose.model('URLModel', URLSchema);
  </code></pre>
  
  <p>After creating new instances of URLModel and saving them in the database I would like to perform the following operation that causes the error:</p>
  
  <pre><code>var docCount = URLModel.estimatedDocumentCount();
  </code></pre>
  
  <p>I very much appreciate any help!
  You can find the link to the full project here:
  <a href="https://glitch.com/edit/#!/stormy-bagel" rel="nofollow noreferrer">https://glitch.com/edit/#!/stormy-bagel</a></p>