keyForPolymorphicForeignKeyId(relationshipName) {
    return `${camelize(relationshipName)}Id`;
  }
Polymorphic relationships are represented with type-id pairs.

    Given the following model

    ```js
    Model.extend({
      commentable: belongsTo({ polymorphic: true })
    });
    ```

    the default Serializer would produce

    ```js
    {
      comment: {
        id: 1,
        commentableType: 'post',
        commentableId: '1'
      }
    }
    ```

    This hook controls how the `id` field (`commentableId` in the above example)
    is serialized. By default it camelizes the relationship and adds `Id` as a suffix.

    @method keyForPolymorphicForeignKeyId
    @param {String} relationshipName
    @return {String}
    @public