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 lt Skill gt  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

User · Accepted Answer

One-to-Many    One Person Has Many Skills  a Skill is not reused between Person s    Unidirectional     A Person can directly reference Skills via its Set Bidirectional      Each  child  Skill has a single pointer back up to the Person  which is not shown in your code    Many-to-Many    One Person Has Many Skills  a Skill is reused between Person s    Unidirectional     A Person can directly reference Skills via its Set Bidirectional      A Skill has a Set of Person s  which relate to it    In a One-To-Many relationship  one object is the  parent  and one is the  child    The parent controls the existence of the child    In a Many-To-Many  the existence of either type is dependent on something outside the both of them  in the larger application context    Your subject matter  domain  should dictate whether or not the relationship is One-To-Many or Many-To-Many -- however  I find that making the relationship unidirectional or bidirectional is an engineering decision that trades off memory  processing  performance  etc   What can be confusing is that a Many-To-Many Bidirectional relationship does not need to be symmetric   That is  a bunch of People could point to a skill  but the skill need not relate back to just those people   Typically it would  but such symmetry is not a requirement  Take love  for example -- it is bi-directional   I-Love    Loves-Me    but often asymmetric   I love her  but she doesn t love me     All of these are well supported by Hibernate and JPA   Just remember that Hibernate or any other ORM doesn t give a hoot about maintaining symmetry when managing bi-directional many-to-many relationships   thats all up to the application

User · Answer

1  The circles are Entities POJOs Beans  2  deg is an abbreviation for degree as in graphs  number of edges    PK Primary key  FK Foreign key  Note the contradiction between the degree and the name of the side  Many corresponds to degree 1 while One corresponds to degree  1

User · Answer

Take a look at this article  Mapping Object Relationships     There are two categories of object relationships that you need to be concerned with when mapping   The first category is based on multiplicity and it includes three types     One-to-one relationships   This is a relationship where the maximums of each of its multiplicities is one  an example of which is holds relationship between Employee and Position in Figure 11   An employee holds one and only one position and a position may be held by one employee  some positions go unfilled    One-to-many relationships  Also known as a many-to-one relationship  this occurs when the maximum of one multiplicity is one and the other is greater than one   An example is the works in relationship between Employee and Division   An employee works in one division and any given division has one or more employees working in it   Many-to-many relationships  This is a relationship where the maximum of both multiplicities is greater than one  an example of which is the assigned relationship between Employee and Task   An employee is assigned one or more tasks and each task is assigned to zero or more employees        The second category is based on   directionality and it contains two   types  uni-directional relationships   and bi-directional relationships     Uni-directional relationships   A uni-directional relationship when an object knows about the object s  it is related to but the other object s  do not know of the original object   An example of which is the holds relationship between Employee and Position in Figure 11  indicated by the line with an open arrowhead on it   Employee objects know about the position that they hold  but Position objects do not know which employee holds it  there was no requirement to do so    As you will soon see  uni-directional relationships are easier to implement than bi-directional relationships   Bi-directional relationships   A bi-directional relationship exists when the objects on both end of the relationship know of each other  an example of which is the works in relationship between Employee and Division   Employee objects know what division they work in and Division objects know what employees work in them

User · Answer

I would explain that way   OneToOne - OneToOne relationship   OneToOne Person person    OneToOne Nose nose    OneToMany - ManyToOne relationship   OneToMany Shepherd gt  shepherd    ManyToOne List lt Sheep gt  sheeps    ManyToMany - ManyToMany relationship   ManyToMany List lt Traveler gt  travelers    ManyToMany List lt Destination gt  destinations

User · Answer

this would probably call for a many-to-many relation ship as follows      public class Person       private Long personId       manytomany      private Set skills        Getters and setters    public class Skill      private Long skillId      private String skillName       manyToMany MappedBy  skills targetClass  Person       private Set persons      people would not be a good convenion        Getters and setters        you may need to define a joinTable   JoinColumn but it will possible work also without

User · Answer

First of all  read all the fine print   Note that NHibernate  thus  I assume  Hibernate as well  relational mapping has a funny correspondance with DB and object graph mapping   For example  one-to-one relationships are often implemented as a many-to-one relationship   Second  before we can tell you how you should write your O R map  we have to see your DB as well   In particular  can a single Skill be possesses by multiple people   If so  you have a many-to-many relationship  otherwise  it s many-to-one   Third  I prefer not to implement many-to-many relationships directly  but instead model the  join table  in your domain model--i e   treat it as an entity  like this   class PersonSkill        Person person      Skill skill          Then do you see what you have   You have two one-to-many relationships    In this case  Person may have a collection of PersonSkills  but would not have a collection of Skills    However  some will prefer to use many-to-many relationship  between Person and Skill   this is controversial   Fourth  if you do have bidirectional relationships  e g   not only does Person have a collection of Skills  but also  Skill has a collection of Persons   NHibernate does not enforce bidirectionality in your BL for you  it only understands bidirectionality of the relationships for persistence purposes   Fifth  many-to-one is much easier to use correctly in NHibernate  and I assume Hibernate  than one-to-many  collection mapping    Good luck

User · Answer

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  quot A State has Cities quot   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 lt Skill gt  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   quot A User has Addresses quot   In this case we might have three columns address 1 id  address 2 id and address 3 id or a look up table with multi column unique constraint on  user id on address id    In unidirectional  a User will have Address address  Bidirectional will have an additional List lt User gt  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

User · Answer

One-to-many The one-to-many table relationship looks like this   In a relational database system  a one-to-many table relationship associates two tables based on a Foreign Key column in the child table referencing the Primary Key of one record in the parent table  In the table diagram above  the post id column in the post comment table has a Foreign Key relationship with the post table id Primary Key column      ALTER TABLE         post comment     ADD CONSTRAINT         fk post comment post id     FOREIGN KEY  post id  REFERENCES post   ManyToOne annotation In JPA  the best way to map the one-to-many table relationship is to use the  ManyToOne annotation  In our case  the PostComment child entity  maps the post id Foreign Key column using the  ManyToOne annotation       Entity name    quot PostComment quot        Table name    quot post comment quot       public class PostComment                 Id          GeneratedValue         private Long id               private String review                ManyToOne fetch   FetchType LAZY          private Post post                  Using the JPA  OneToMany annotation Just because you have the option of using the  OneToMany annotation  it doesn t mean it should be the default option for all the one-to-many database relationships  The problem with JPA collections is that we can only use them when their element count is rather low  The best way to map a  OneToMany association is to rely on the  ManyToOne side to propagate all entity state changes       Entity name    quot Post quot        Table name    quot post quot       public class Post                 Id          GeneratedValue         private Long id               private String title                OneToMany              mappedBy    quot post quot                cascade   CascadeType ALL               orphanRemoval   true                   private List lt PostComment gt  comments   new ArrayList lt  gt                    Constructors  getters and setters removed for brevity              public void addComment PostComment comment                comments add comment               comment setPost this                          public void removeComment PostComment comment                comments remove comment               comment setPost null                    The parent Post entity features two utility methods  e g  addComment and removeComment  which are used to synchronize both sides of the bidirectional association  You should provide these methods whenever you are working with a bidirectional association as  otherwise  you risk very subtle state propagation issues  The unidirectional  OneToMany association is to be avoided as it s less efficient than using  ManyToOne or the bidirectional  OneToMany association  One-to-one The one-to-one table relationship looks as follows   In a relational database system  a one-to-one table relationship links two tables based on a Primary Key column in the child which is also a Foreign Key referencing the Primary Key of the parent table row  Therefore  we can say that the child table shares the Primary Key with the parent table  In the table diagram above  the id column in the post details table has also a Foreign Key relationship with the post table id Primary Key column      ALTER TABLE         post details     ADD CONSTRAINT         fk post details id     FOREIGN KEY  id  REFERENCES post  Using the JPA  OneToOne with  MapsId annotations The best way to map a  OneToOne relationship is to use  MapsId  This way  you don t even need a bidirectional association since you can always fetch the PostDetails entity by using the Post entity identifier  The mapping looks like this   Entity name    quot PostDetails quot    Table name    quot post details quot   public class PostDetails         Id     private Long id        Column name    quot created on quot       private Date createdOn        Column name    quot created by quot       private String createdBy        OneToOne fetch   FetchType LAZY       MapsId      JoinColumn name    quot id quot       private Post post       public PostDetails           public PostDetails String createdBy            createdOn   new Date            this createdBy   createdBy               Getters and setters omitted for brevity    This way  the id property serves as both Primary Key and Foreign Key  You ll notice that the  Id column no longer uses a  GeneratedValue annotation since the identifier is populated with the identifier of the post association  Many-to-many The many-to-many table relationship looks as follows   In a relational database system  a many-to-many table relationship links two parent tables via a child table which contains two Foreign Key columns referencing the Primary Key columns of the two parent tables  In the table diagram above  the post id column in the post tag table has also a Foreign Key relationship with the post table id Primary Key column      ALTER TABLE         post tag     ADD CONSTRAINT         fk post tag post id     FOREIGN KEY  post id  REFERENCES post  And  the tag id column in the post tag table has a Foreign Key relationship with the tag table id Primary Key column      ALTER TABLE         post tag     ADD CONSTRAINT         fk post tag tag id     FOREIGN KEY  tag id  REFERENCES tag  Using the JPA  ManyToMany mapping This is how you can map the many-to-many table relationship with JPA and Hibernate       Entity name    quot Post quot        Table name    quot post quot       public class Post             Id          GeneratedValue         private Long id           private String title            ManyToMany cascade                  CascadeType PERSIST               CascadeType MERGE                     JoinTable name    quot post tag quot               joinColumns    JoinColumn name    quot post id quot                inverseJoinColumns    JoinColumn name    quot tag id quot                     private Set lt Tag gt  tags   new HashSet lt  gt                Getters and setters ommitted for brevity          public void addTag Tag tag                tags add tag               tag getPosts   add this                      public void removeTag Tag tag                tags remove tag               tag getPosts   remove this                       Override         public boolean equals Object o                if  this    o  return true              if    o instanceof Post   return false              return id    null  amp  amp  id equals   Post  o  getId                         Override         public int hashCode                 return getClass   hashCode                          Entity name    quot Tag quot        Table name    quot tag quot       public class Tag             Id          GeneratedValue         private Long id            NaturalId         private String name            ManyToMany mappedBy    quot tags quot           private Set lt Post gt  posts   new HashSet lt  gt                Getters and setters ommitted for brevity           Override         public boolean equals Object o                if  this    o  return true              if  o    null    getClass      o getClass    return false              Tag tag    Tag  o              return Objects equals name  tag name                       Override         public int hashCode                 return Objects hash name                     The tags association in the Post entity only defines the PERSIST and MERGE cascade types  The REMOVE entity state transition doesn t make any sense for a  ManyToMany JPA association since it could trigger a chain deletion that would ultimately wipe both sides of the association  The add remove utility methods are mandatory if you use bidirectional associations so that you can make sure that both sides of the association are in sync  The Post entity uses the entity identifier for equality since it lacks any unique business key  You can use the entity identifier for equality as long as you make sure that it stays consistent across all entity state transitions  The Tag entity has a unique business key which is marked with the Hibernate-specific  NaturalId annotation  When that s the case  the unique business key is the best candidate for equality checks  The mappedBy attribute of the posts association in the Tag entity marks that  in this bidirectional relationship  the Post entity owns the association  This is needed since only one side can own a relationship  and changes are only propagated to the database from this particular side  The Set is to be preferred  as using a List with  ManyToMany is less efficient

[java] Difference Between One-to-Many, Many-to-One and Many-to-Many?

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