title: Api-platform: how to output related entity id instead of resource path?
body: <p>How can I get the related entity Id instead of resource path?</p>
  
  <p>For example, here's what I get now:</p>
  
  <pre><code>{
    "id": "/api/articles/0d8cb40c-221b-4c54-9e29-43877093b839",
    "type": "Article",
    "attributes": {
      "_id": "0d8cb40c-221b-4c54-9e29-43877093b839",
      "title": "Article",
      "content": "&lt;p&gt;Lorem ipsum dolor sit amet, consectetur adipiscing elit.&lt;/p&gt;",
      "dateCreated": "2018-07-17T18:20:29+03:00",
      "dateUpdated": "2018-07-17T18:20:29+03:00"
    },
    "relationships": {
      "author": {
        "data": {
          "type": "User",
          "id": "/api/users/b26e0381-6800-4f47-b269-9a92b27ad331"
        }
      }
    }
  }
  </code></pre>
  
  <p>I would like to get related entity id like this:</p>
  
  <pre><code>"relationships": {
  "author": {
    "data": {
      "type": "User",
      "id": "b26e0381-6800-4f47-b269-9a92b27ad331"
    }
  }
  </code></pre>