Mongoose populate not populating
<p>I am trying to populate my users car inventory. All the cars have a userId attached to them when they are created but when I go to populate the inventory it doesn't work and I get no errors.</p>

<p>Here are my models:</p>

<p><strong>User.js</strong></p>

<pre><code>let UserSchema = mongoose.Schema({
  username: {
    type: String,
    required: true,
    unique: true
  },
  password: {
    type: String,
    required: true
  },
  inventory: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Car' }]
});

let User = mongoose.model('User', UserSchema);
models.User = User;
</code></pre>

<p><strong>Cars.js</strong></p>

<pre><code>let CarSchema = mongoose.Schema({
  userId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'
  },
  make: {
    type: String,
    required: true
  },
  model: {
    type: String,
    required: true
  },
  year: {
    type: String
  }
});

let Car = mongoose.model('Car', CarSchema);
models.Car = Car;
</code></pre>

<p>Here is the populate code:</p>

<pre><code>router.route('/users/:user/inventory').get((req, res) =&gt; {
    User.findById(userId)
      .populate('inventory') 
      .exec((err, user) =&gt; {
        if (err) {
          console.log("ERRROORRR " + err)
          return res.send(err);
        }

        console.log('Populate ' + user)
        res.status(200).json({message: 'Returned User', data: user});
      });
    });
  };
</code></pre>

<p>This is what a car object looks like in the database:</p>

<pre><code>{
  "_id": ObjectId("5759c00d9928cb581b5424d0"),
  "make": "dasda",
  "model": "dafsd",
  "year": "asdfa",
  "userId": ObjectId("575848d8d11e03f611b812cf"),
  "__v": 0
}
</code></pre>

<p>Any advice would be great! Thanks!</p>