keyForForeignKey(relationshipName) {
    return `${camelize(relationshipName)}Id`;
  }
Like `keyForRelationshipIds`, but for `belongsTo` relationships.

    For example, if you're serializing a `blogPost` that sideloads one `author`,
    your `blogPost` JSON would include a `authorId` key:

    ```
    {
      blogPost: {
        id: 1,
        authorId: 1
      },
      author: ...
    }
    ```

    Overwrite `keyForForeignKey` to format this key:

    ```js
    // serializers/application.js
    export default Serializer.extend({
      keyForForeignKey(relationshipName) {
        return underscore(relationshipName) + '_id';
      }
    });
    ```

    Now the response will look like:

    ```js
    {
      blogPost: {
        id: 1,
        author_id: 1
      },
      author: ...
    }
    ```

    @method keyForForeignKey
    @param relationshipName
    @public