I'm not sure what I'm missing, but I'm getting an empty object back from the Element API for the summary and/or body for any given entry.

elementapi.php

...
'transformer' => function (EntryModel $entry) {
    return [
        'title' => $entry->title,
        'url' => $entry->url,
        'summary' => $entry->summary
    ];
},
...

Here is the json I am getting back:

{"data":[{"title":"My Title","url":"http:\/\/local.mysite.dev\/section\/my-entry","summary":{}}