七种映射关系

直到现在我对映射关系都不是非常了解,希望以后有个更深的了解,今天只是把这七种关系记下来.

One-to-one unidirectional

The relationship between a customer and an address. You clearly want to be able to look up a customer's address, but you probably don't care about looking up an address's customer.

One-to-one bidirectional

The relationship between a customer and a credit card number. Given a customer, you obviously want to be able to look up his credit card number. Given a credit card number, it is also conceivable that you would want to look up the customer who owns the credit card.

One-to-many unidirectional

The relationship between a customer and a phone number. A customer can have many phone numbers (business, home, cell, etc.). You might need to look up a customer's phone number, but you probably wouldn't use one of those numbers to look up the customer.

One-to-many bidirectional

The relationship between a cruise and a reservation. Given a reservation, you want to be able to look up the cruise for which the reservation was made. And given a particular cruise, you want to be able to look up all reservations. (Note that a many-to-one bidirectional relationship is just another perspective on the same concept.)

Many-to-one unidirectional

The relationship between a cruise and a ship. You want to be able to look up the ship that will be used for a particular cruise, and many cruises share the same ship, though at different times. It's less useful to look up the ship to see which cruises are associated with it, although if you want this capability, you can implement a many-to-one bidirectional relationship.

Many-to-many unidirectional

The relationship between a reservation and a cabin. It's possible to make a reservation for multiple cabins, and you clearly want to be able to look up the cabin assigned to a reservation. However, you're not likely to want to look up the reservation associated with a particular cabin. (If you think you need to do so, implement it as a bidirectional relationship.)

Many-to-many bidirectional

The relationship between a cruise and a customer. A customer can make reservations on many cruises, and each cruise has many customers. You want to be able to look up both the cruises on which a customer has a booking and the customers that will be going on any given cruise.

摘自

Enterprise JavaBeans, 3.0