在Project模型中：
has_and_belongs_to_many :subprojects,
:join_table => "project_projects"
在 Cucumber 功能文件中：
Given the following projects exist:
| name | type |
| first | foo |
| second | bar |
| third | baz |
| fourth | bar |
Given the following subprojects exist:
| subproject | superproject |
| first | |
| second | first |
| third | first |
| third | second |
...
And I add a superproject "second" to "fourth"
And I add a superproject "third" to "fourth"
换句话说，third有两个父级，first并且second和fourth最初没有父级，但是为了单个场景的目的添加了一些父级。我们如何在 Factory Girl Rails 中为此创建工厂？我们已经有一个:project工厂，但是没有Project_project模型，原因很明显，而且我们显然不想在每次添加关系时都创建新项目——我们需要重用子项目<->超级项目工厂中的现有对象。