什么是单向多对多关联？

Question

As far as I understand, Many to Many association between 2 entities A and B mean that A can have more than one values in B and B can have more than one values in A .

If that is correct what does unidirectional many to many association mean ? How is it different from bidirectional many to many mapping ?

As the name suggests many to many means bidirectional !

Answer 1

I think you are getting confused with database tables and its corresponding entities.

From DB perspective: Many to many would be implemented using third table, which holds entries from both the tables.

From java entities perspective: its upto you, if you want it to be bidirectional or unidirectional. What it simple means, is that, you can have a collection of Students in Module, but you may not want to care, to how many Modules student belongs to.

class Student{
//attributes and methods
}

class Module{

Collection<Student>  students;

//other attributes and methods.

}

Now, being in administration of some institution, I want to have flexibility of creating modules and assigning students to this module. I would prefer many-many unidirectional. So, here actually if you see, there are many modules as well as many students( i hope you can make out what i mean). Having said that, you are not restricted to find out if a Student belongs to Java module or not. I hope it makes it little more clear.

Answer 2

Unidirectional means that, for example, A has a reference to B s, but not the other way around.

many to many means bidirectional No . The many to many concept is for database table mapping. Hibernate uses it (and other x-to-x ) to generate the proper database table schema. The x-directional concept is for java objects.