Difference Between One-to-Many, Many-to-One and Many-to-Many?

Question

Ok so this is probably a trivial question but I'm having trouble visualizing and understanding the differences and when to use each. I'm also a little unclear as to how concepts like uni-directional and bi-directional mappings affect the one-to-many/many-to-many relationships. I'm using Hibernate right now so any explanation that's ORM related will be helpful.

As an example let's say I have the following set-up:

public class Person{
    private Long personId;
    private Set<Skill> skills;
    //Getters and setters
}

public class Skill{
    private Long skillId;
    private String skillName;
    //Getters and setters
}

So in this case what kind of mapping would I have? Answers to this specific example are definitely appreciated but I would also really like an overview of when to use either one-to-many and many-to-many and when to use a join table versus a join column and unidirectional versus bidirectional.

Answer 1

Looks like everyone is answering One-to-many vs. Many-to-many:

The difference between One-to-many, Many-to-one and Many-to-Many is:

One-to-many vs Many-to-one is a matter of perspective. Unidirectional vs Bidirectional will not affect the mapping but will make difference on how you can access your data.

• In Many-to-one the many side will keep reference of the one side. A good example is "A State has Cities". In this case State is the one side and City is the many side. There will be a column state_id in the table cities.

In unidirectional, Person class will have List<Skill> skills but Skill will not have Person person. In bidirectional, both properties are added and it allows you to access a Person given a skill( i.e. skill.person).

• In One-to-Many the one side will be our point of reference. For example, "A User has Addresses". In this case we might have three columns address_1_id, address_2_id and address_3_id or a look up table with unique constraint on user_id and address_id.

In unidirectional, a User will have Address address. Bidirectional will have an additional List<User> users in the Address class.

• In Many-to-Many members of each party can hold reference to arbitrary number of members of the other party. To achieve this a look up table is used. Example for this is the relationship between doctors and patients. A doctor can have many patients and vice versa.