keyForModel(modelName) {
    return dasherize(modelName);
  }
By default, JSON:API's linkage data is only added for relationships that are being included in the current request.

      That means given an `author` model with a `posts` relationship, a GET request to /authors/1 would return a JSON:API document with an empty `relationships` hash:

      ```js
      {
        data: {
          type: 'authors',
          id: '1',
          attributes: { ... }
        }
      }
      ```

      but a request to GET /authors/1?include=posts would have linkage data added (in addition to the included resources):

      ```js
      {
        data: {
          type: 'authors',
          id: '1',
          attributes: { ... },
          relationships: {
            data: [
              { type: 'posts', id: '1' },
              { type: 'posts', id: '2' },
              { type: 'posts', id: '3' }
            ]
          }
        },
        included: [ ... ]
      }
      ```

      To add the linkage data for all relationships, you could set `alwaysIncludeLinkageData` to `true`:

      ```js
      JSONAPISerializer.extend({
        alwaysIncludeLinkageData: true
      });
      ```

      Then, a GET to /authors/1 would respond with

      ```js
      {
        data: {
          type: 'authors',
          id: '1',
          attributes: { ... },
          relationships: {
            posts: {
              data: [
                { type: 'posts', id: '1' },
                { type: 'posts', id: '2' },
                { type: 'posts', id: '3' }
              ]
            }
          }
        }
      }
      ```

      even though the related `posts` are not included in the same document.

      You can also use the `links` method (on the Serializer base class) to add relationship links (which will always be added regardless of the relationship is being included document), or you could use `shouldIncludeLinkageData` for more granular control.

      For more background on the behavior of this API, see [this blog post](http://www.ember-cli-mirage.com/blog/changing-mirages-default-linkage-data-behavior-1475).

      @property alwaysIncludeLinkageData
      @type {Boolean}
      @public