title: Invalid schema configuration: `model` is not a valid type within the array `characters`
body: <p>I'm trying to create a schema subdocument but am getting the error listed above, 
  The schemas in question look like this 
  <strong>Schema casuing issues</strong></p>
  
  <pre><code>const mongoose = require('mongoose');
  const Schema = mongoose.Schema
  
  const CharacterSchema = new Schema();
  CharacterSchema.add({
      name: {
          type: String,
          required: true
      },
      title: {
          type: String
      },
      charcterClass: { // will be limited in form creation
          type: String
      },
      level: {
          type: Number
      }
  });
  
  const Charcter = mongoose.model('User', CharacterSchema);
  module.exports = Charcter;
  </code></pre>
  
  <p><strong>Schema calling schema above</strong></p>
  
  <pre><code>const mongoose = require ('mongoose');
  const Schema = mongoose.Schema;
  const {CharacterSchema} = require(__dirname +'/CharacterModel.js');
  
  const UserSchema = new Schema()
  UserSchema.add({
      name: {
          type: String, 
          required: true
      } ,
      characters: [CharacterSchema]
  });
  
  const User = mongoose.model('Character', UserSchema);
  module.exports = User;
  </code></pre>