View source for EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping/Relationship Mappings/Collection Mappings/ManyToMany

{{EclipseLink_UserGuide
|info=y
|toc=n
|eclipselink=y
|eclipselinktype=JPA
|api=y
|apis=
*[http://www.eclipse.org/eclipselink/api/latest/javax/persistence/ManyToMany.html @ManyToMany]
| examples=y
|example=
*[[EclipseLink/Examples/JPA/MappingSelectionCriteria|Mapping with Selection Criteria]]
}}
=Many-to-Many Mapping=
Many-to-many mappings represent the relationships between a collection of source objects and a collection of target objects. They require the creation of an intermediate table for managing the associations between the source and target records.

This figure illustrates a many-to-many mapping in Java and in relational database tables.


<span id="Figure 32-5"></span>
''''' Many-to-many Relationships'''''

[[Image:mmmapfig.gif|Many-to-many Relationships]]

{{EclipseLink_Note
|note=For the <tt>projects</tt> attribute shown in the [[#Figure 32-5|Many-to-many Relationships]] you can use a <tt>Collection</tt> interface (or any class that implements the <tt>Collection</tt> interface) for declaring the collection attribute. }}

=JPA Mapping=
By default, JPA automatically defines a <tt>ManyToMany</tt> mapping for a many-valued association with many-to-many multiplicity.

Use the <tt>@ManyToMany</tt> annotation to do the following:
* configure the [[EclipseLink/UserGuide/JPA/Basic_JPA_Development/Mapping/Relationship_Mappings/Common_Relationship_Configurations/Fetch_Type|Fetch Type]] to <tt>EAGER</tt><nowiki>;</nowiki>
* configure the mapping to forbid null values (for nonprimitive types) in case <tt>null</tt> values are inappropriate for your application;
* configure the associated target entity because the <tt>Collection</tt> used is not defined using generics;
* configure the operations that must be cascaded to the target of the association (for example, if the owning entity is removed, ensure that the target of the association is also removed).

{{EclipseLink_AttributeTable
|caption=@ManyToMany Annotation Attributes
|content=<tr>
 <td>'''<tt>cascade</tt>'''</td>
 <td> – By default, JPA does not cascade any persistence operations to the target of the association.If you want some or all persistence operations cascaded to the target of the association, set the value of this attribute to one or more <tt>CascadeType</tt> instances, including the following:
* <tt>ALL</tt> – Any persistence operation performed on the owning entity is cascaded to the target of the association.
* <tt>MERGE</tt> – If the owning entity is merged, the merge is cascaded to the target of the association.
* <tt>PERSIST</tt> – If the owning entity is persisted, the persist is cascaded target of the association.
* <tt>REFRESH</tt> – If the owning entity is refreshed, the refresh is cascaded target of the association.
* <tt>REMOVE</tt> – If the owning entity is removed, the target of the association is also removed.</td>
 <td>An empty <tt>javax.persitence.CascadeType</tt> array</td>
 <td>No</td>
</tr>
<tr>
 <td>'''<tt>fetch</tt>'''</td>
 <td> – By default, EclipseLink persistence provider uses a [[EclipseLink/UserGuide/JPA/Basic_JPA_Development/Mapping/Relationship_Mappings/Common_Relationship_Configurations/Fetch_Type|fetch type]] of <tt>javax.persitence.FetchType.LAZY</tt><nowiki>: this is a hint to the persistence provider that data should be fetched lazily when it is first accessed (if possible).</nowiki><br>If the default is inappropriate for your application or a particular persistent field, set <tt>fetch</tt> to <tt>FetchType.EAGER</tt><nowiki>: this is a requirement on the persistence provider runtime that data must be eagerly fetched.</nowiki>.</td>
 <td><tt>javax.persitence.FetchType.LAZY</tt></td>
 <td>No</td>
</tr>
<tr>
 <td>'''<tt>mappedBy</tt>'''</td>
 <td>By default, if the relationship is unidirectional, EclipseLink persistence provider determines the field that owns the relationship.<br>If the relationship is bidirectional, then set the <tt>mappedBy</tt> element on the inverse (non-owning) side of the association to the name of the field or property that owns the relationship, as the [[#@ManyToMany Annotation - Project Class with Generics|@ManyToMany Annotation - Project Class with Generics]] example shows.</td>
 <td></td>
 <td>No</td>
</tr>
<tr>
 <td>'''<tt>targetEntity</tt>'''</td>
 <td>By default, if you are using a <tt>Collection</tt> defined using generics, then the persistence provider infers the associated target entity from the type of the object being referenced. Thus, the default is the parameterized type of the <tt>Collection</tt> when defined using generics.<br>If your <tt>Collection</tt> does not use generics, then you must specify the entity class that is the target of the association: set the <tt>targetEntity</tt> element on owning side of the association to the <tt>Class</tt> of the entity that is the target of the relationship.</td>
 <td></td>
 <td>No</td>
</tr>}}

The [[#@ManyToMany Annotation - Employee Class with Generics|@ManyToMany Annotation - Employee Class with Generics]] example and the [[#@ManyToMany Annotation - Project Class with Generics|@ManyToMany Annotation - Project Class with Generics]] example show how to use this annotation to configure a many-to-many mapping between <tt>Employee</tt> and <tt>Project</tt> using generics.


<span id="@ManyToMany Annotation - Employee Class with Generics"></span>
======''Example: @ManyToMany Annotation - Employee Class with Generics''======
<source lang="java">
 @Entity
 public class Employee implements Serializable {
     @Id
     private int id;
     private String name;
     @ManyToMany
     @JoinTable(name="EMP_PROJ",
                 joinColumns=
                      @JoinColumn(name="EMP_ID"),
                 inverseJoinColumns=
                      @JoinColumn(name="PROJ_ID")
     )
     private Collection<Project> projects;
     ...
 }
</source>

{{EclipseLink_Note
|note=Use a <tt>@JoinTable</tt> annotation to define a many-to-many join table; if you do not specify this annotation, EclipseLink will default to <tt>@JoinTable</tt> with the join table name format of <tt><source-table-name>_<target-table-name></tt> in uppercase characters, and with columns format of <tt><source-entity-alias>_<source-primary-key-column></tt>, <tt><source-field-name>_ <target-primary-key-column></tt> (or <source-property-name>_ <target-primary-key-column>) in uppercase characters.

{{EclipseLink_Spec|section=Section 11.1.23 "JoinTable Annotation"}}
}}

<br>

<span id="@ManyToMany Annotation - Project Class with Generics"></span>
======''Example: @ManyToMany Annotation - Project Class with Generics''======
<source lang="java">
 @Entity
 public class Project implements Serializable {
     ...
     @ManyToMany(mappedBy="projects")
     public Set<Employee> getEmployees() { 
         return employees;
     }
     ...
 }
</source>


{{EclipseLink_Spec|section=Section 11.1.25 "ManyToMany Annotation"}}

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