employee 1可能有很多下屬(employee2, employee3, employee4)
In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:
With this setup, you can retrieve @employee.subordinates and @employee.manager.
In your migrations/schema, you will add a references column to the model itself.