GET /lists won't sideload

[jrhorn424]

I've got a lists model, which you can see here: https://github.com/ga-wdi-boston/ember-example-listr-api/blob/983b9ee41c7c266acc8780b3b5dedd3a2cdaefca/app/models/list.js

The associated item model is also pushed.

When I get the lists, their items are not sideloaded. Instead, I receive the following:

{
  "lists": [
    {
      "id": 1,
      "title": "Favorite Things",
      "hidden": false,
      "createdAt": "2015-12-11T19:32:04.065Z",
      "updatedAt": "2015-12-11T19:32:04.065Z"
    },
    {
      "id": 2,
      "title": "Todo",
      "hidden": false,
      "createdAt": "2015-12-11T19:32:04.065Z",
      "updatedAt": "2015-12-11T19:32:04.065Z"
    }
  ]
}

Is there a configuration option I missed?