keyForRelationship(key) {
    return dasherize(key);
  }
Used to customize the key for a relationships. By default, compound relationship names are dasherized.

    For example, the JSON:API document for an `author` model with a `blogPosts` relationship would be:

    ```js
    {
      data: {
        id: 1,
        type: 'author',
        attributes: {
          ...
        },
        relationships: {
          'blog-posts': {
            ...
          }
        }
      }
    }
    ```

    @method keyForRelationship
    @param {String} key
    @return {String}
    @public