Java Persistence/manytomany

A ManyToMany relationship in Java are where the source object has an attribute that stores a collection of target objects and (i f) Those target objects had the inverse relationship back to the source object it would also is a ManyToMany relationship. All relationships in Java and JPA was unidirectional, in so if a source object references a target object there is no Gu Arantee that the target object also have a relationship to the source object. This was different than a relational database, in which relationships be defined through foreign keys and querying such th At the inverse query always exists.

JPA also defines a OneToMany relationship, which is similar to a ManyToMany relationship except that the inverse relationship (if I T were defined) is a ManyToOne relationship. The main difference between a and a relationship in JPA are that a all makes use of OneToMany ManyToMany ManyToMany a intermediate rela tional join table to store the relationship, where as a OneToMany can either with a join table, or a foreign key in target Obje CT ' s table referencing the Source object table ' s primary key.

In JPA a ManyToMany relationship is defined through the @ManyToMany annotation or the <many-to-many> element.

All ManyToMany relationships require a JoinTable . The IS JoinTable defined using the @JoinTable annotation and <join-table> XML element. JoinTablethe defines a foreign key to the source object ' s primary key ( joinColumns ), and a foreign key to the target object ' s Prima Ry Key ( inverseJoinColumns ). Normally the primary key of the is the JoinTable combination of both foreign keys.

Example of a manytomany relationship database[edit]

EMPLOYEE (table)

Id	FIRSTNAME	LASTNAME
1	Bob	The
2	Sarah	Smith

Emp_proj (table)

emp_id	proj_id
1	1
1	2
2	1

PROJECT (table)

Id	NAME
1	Gis
2	Sig

Example of a manytomany relationship annotation[edit]

@EntityPublicClassEmployee{@Id@Column(Name="ID")PrivateLongId;...@ManyToMany@JoinTable(Name="Emp_proj"joincolumns= @JoinColumn  span class= "n" >name= "emp_id"  Referencedcolumnname= "ID" ), inversejoincolumns< Span class= "o" >= @JoinColumn  (name=  "proj_id" referencedcolumnname=  "ID" ) private list<project> span class= "n" >projects              /span>                

Example of a manytomany relationship XML[edit]

<entityName="Employee"class="Org.acme.Employee"access="FIELD"><attributes><idName="id"><columnName="Emp_id"/></id><set name= "projects" table= "emp_proj" lazy= "true"  Cascade= "none" sort= "natural" optimistic-lock=  "false" ><key column= " emp_id "not-null=" true "/><many-to-many class= "Com.flipswap.domain.Project" column= " PROJ_ID "/></set></attributes> </ENTITY>              

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Java Persistence/manytomany