keyForRelationshipIds(relationshipName) {
    return `${this._container.inflector.singularize(
      camelize(relationshipName)
    )}Ids`;
  }
Use this hook to format the key for the IDS of a `hasMany` relationship
    in this model's JSON representation.

    For example, if you're serializing an `author` that
    sideloads many `blogPosts`, by default your `author` JSON would include a `blogPostIds` key:

    ```
    {
      author: {
        id: 1,
        blogPostIds: [1, 2, 3]
      },
      blogPosts: [...]
    }
    ```

    Overwrite `keyForRelationshipIds` to format this key:

    ```js
    // serializers/application.js
    export default Serializer.extend({
      keyForRelationshipIds(relationship) {
        return underscore(relationship) + '_ids';
      }
    });
    ```

    Now the response will look like:

    ```
    {
      author: {
        id: 1,
        blog_post_ids: [1, 2, 3]
      },
      blogPosts: [...]
    }
    ```

    @method keyForRelationshipIds
    @param modelName
    @public