View source for Editing EclipseLink/UserGuide/sandbox/gelernter/Extensible Domain Models

{{EclipseLink_UserGuide 
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|eclipselink=y
|eclipselinktype=JPA
|api=y
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* [http://www.eclipse.org/eclipselink/api/latest/org/eclipse/persistence/annotations/VirtualAccessMethods.html @VirtualAccessMethods]
|examples=y
|example=
*[[EclipseLink/Examples/JPA/Extensibility|Extensible Entities]]
}}
===******SANDBOX VERSION******===
=Extensible Entities=
''This topic is currently under review.''

{{EclipseLink_NewIn
|version=2.3}}
<br><br>.''

Use the <tt>@VirtualAccessMethods</tt> annotation to specify that an entity is extensible. By using virtual properties in an extensible entity, you can specify mappings external to the entity. This allows you to modify the mappings without modifying the entity source file and without redeploying the entity's persistence unit. 

Extensible entities are useful in a multi-tenant (or Software-as-a-Service) environment where a shared, generic application can be used by multiple clients (tenants). Tenants have private access to their own data, as well as to data shared with other tenants. See also [[EclipseLink/UserGuide/JPA/Advanced_JPA_Development/Single-Table_Multi-Tenancy |Single-Table Multi-Tenancy]].

Using extensible entities, you can:

* Build an application where some mappings are common to all users and some mappings are user-specific.
* Add mappings to an application after it is made available to a customer (even post-deployment).
* Use the same <tt>EntityManagerFactory</tt> to work with data after mappings have changed.
* Provide an additional source of metadata to be used by an application. 

To create and support an extensible entity, 

#Configure the entity. See [[#Configuring the Entity| Configuring the Entity]]. 
# Include flexible columns in the database table to store the additional data. See [[#Designing the Schema| Designing the Schema]].
# Specify extended mappings in the <tt>eclipselink-orm.xml</tt> file. See [[#Providing Additional Mappings| Providing Additional Mappings]]
# Configure persistence.xml. See [[#Configuring persistence.xml|Configuring persistence.xml]].

===Configuring the Entity===
To configure the entity,

# [[#Annotate with @VirtualAccessMethods| Annotate with <tt>@VirtualAccessMethods</tt>]]
#  [[#Add get() and set() Methods| Add <tt>get()</tt> and <tt>set()</tt> methods]]
# [[#Add a Data Structure|Add a data structure]]

====Annotate with @VirtualAccessMethods====

Annotate the entity with <tt>@VirtualAccessMethods</tt> to specify that it is extensible and to define virtual properties. 
{{EclipseLink_AttributeTable
|caption=@VirtualAccessMethods Attributes
|content=
<tr>
<td><tt>get</tt></td>
<td> The name of the getter method to use for the virtual property This method must take a single <tt>java.lang.String</tt> parameter and return a <tt>java.lang.Object</tt>. </td>
<td><tt>get</tt></td>
<td>Yes</td>
</tr>
<tr>
<td>'''<tt>set</tt>'''</td>
<td>The name of the setter method to use for the virtual property This method must take a <tt>java.lang.String</tt> parameter and return a <tt>java.lang.Object</tt> parameter.</td>
<td><tt>set</tt></td>
<td>Yes</td>
</tr>
}}
<br>

==== Add get() and set() Methods====
Add <tt>get(String)</tt> and <tt>set(String, Object)</tt> methods to the entity.

The <tt>get()</tt> method returns a value by property name and the <tt>set()</tt> method stores a value by property name. The default names for these methods are <tt>get</tt> and <tt>set</tt>, and they can be overridden with the <tt>@VirtualAccessMethods</tt> annotation. 

EclipseLink weaves these methods if weaving is enabled, which provides support for lazy loading, change tracking, fetch groups, and internal optimizations. You must use the the <tt>get(String)</tt> and <tt>set(String, Object)</tt> signatures, or else weaving will not work.

{{EclipseLink_Note
|note=Weaving is not supported when using virtual access methods with OneToOne mappings. If attempted, an exception will be thrown.
}}

==== Add a Data Structure ====

Add a data structure to store the extended attributes and values, that is, the virtual mappings. These can then be mapped to the database. See [[#Providing Additional Mappings| Providing Additional Mappings]].

A common way to store the virtual mappings is in a <tt>Map</tt> (as shown in the examples in this topic), but you can use other ways, as well. For example you could store the virtual mappings in a directory system. 

When using field-based access, annotate the data structure with <tt>@Transient</tt> so it cannot use it for another mapping. When using property-based access, <tt>@Transient'</tt> is unnecessary.

====Example====

The following example shows an entity that uses property access. 

<source lang="java">
@Entity
  @VirtualAccessMethods
  public class Customer{
 
    @Id
    private int id;
...
 
    @Transient
    private Map<String, Object> extensions;

    public <T> T get(String name) {
        return (T) extentions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
</source>

===Designing the Schema===

Provide database tables with extra columns for storing flexible mapping data. For example, the following Customer table includes two predefined columns, ID and NAME, and three flexible columns, FLEX_COL1, FLEX_COL2, FLEX_COL3:

*CUSTOMER 
**INTEGER ID 
**VARCHAR NAME 
**VARCHAR FLEX_COL1 
**VARCHAR FLEX_COL2 
**VARCHAR FLEX_CO31 

You can then specify which of those flex columns should be used to persist an extended attribute, as described below, in [[#Providing Additional Mappings| Providing Additional Mappings]].

===Providing Additional Mappings===

To provide additional mappings, add the mappings to the <tt>eclipselink-orm.xml</tt> file, for example:
 
<source lang="xml">
<basic name="idNumber" attribute-type="String">
  <column name="FLEX_COL1"/>
  <access-methods get-method="get" set-method="set"/>
</basic>
</source>

'''''//REVIEWERS: Are there any limitations on the types of mappings that support flexible mappings? Also, do you think anything more should be said about what you have to do in eclipselink-orm.xml?//'''''

* ''' I think that part of this will be addressed by anything we do to document using <access-methods> to specify Virtual mappings'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 16:04, 28 June 2011 (UTC)
* ''' The XML file simply gets treated as another XML file in the list of XML files. As long as you obey all the rules related to what can be overridden, you can use any kind of mapping.  The challenge in using non-virtual mappings is how to have the data structures that support them make sense when the document is not there. e.g. if you're going to have an extension that uses an instance variable, for the instances of the application that don't use that extension file, how is that instance variable treated - JPA will likely try to use it for a mapping using its defaulting-rules''' --[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 16:04, 28 June 2011 (UTC)

===Configuring persistence.xml===

Configure persistence unit properties in <tt>persistence.xml</tt> to indicate that the application should retrieve the flexible mappings from the <tt>eclipselink-orm.xml</tt> file,. For example:

'''''//REVIEWERS Did I get that intro right? Would different wording be better here? The design doc said “Use persistence unit properties to get your application to use the file."//'''''

* '''Both persistence unit propeties and persistence.xml are legitimate use cases.  We should describe both.  persistence.xml allows either a default, or a single-user file that can be changed.  persistence unit properties allow specification of the file at runtime and provides a more dynamic experience.'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 16:06, 28 June 2011 (UTC)

<source lang="xml">
<property name="eclipselink.metadata-source" value="XML"/>
<property name="eclipselink.metadata-source.xml.url" value="foo://bar"/>
</source>

'''''//REVIEWERS What more can be  said about these? See my related questions below, under [[#Configuring the EntityManagerFactory and the Metadata Repository | Configuring the EntityManagerFactory and the Metadata Repository]].//'''''

* '''Maybe the two sections should go together.  We could mention that by default we support using a file at a URL, but it is possible to also override how the repository works and then go into details.'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:20, 28 June 2011 (UTC)

==Examples ==
The following examples illustrate variations on configuring extensible entities.

====Example 1====

''Example 1''  illustrates the following: 
*Field access is used for non-extension fields.
*Virtual access is used for extension fields, using defaults <tt>(get(String)</tt> and <tt>set(String, Object))</tt> .
*The <tt>get(String)</tt> and <tt>set(String, Object)</tt> methods will be woven, even if no mappings use them, because of the presence of <tt>@VirtualAccessMethods</tt>.
*Extensions are mapped in a portable way by specifying <tt>@Transient</tt>. 

Example 1

<source lang="java">
@Entity
  @VirtualAccessMethods
  public class Address {
 
    @Id
    private int id;
 
    @Transient
    private Map<String, Object> extensions;
 
    public int getId(){
        return id;
    }
 
    public <T> T get(String name) {
        return (T) extentions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
 
...
</source>

====Example 2====

Example 2  illustrates the following: 

*Field access is used for non-extension fields.
* The <tt>@VirtualAccessMethods</tt> annotation overrides methods to be used for getting and setting. 
*The <tt>get(String)</tt> and <tt>set(String, Object)</tt> methods will be woven, even if no mappings use them, because of the presence of <tt>@VirtualAccessMethods</tt>.<br>
*Extensions are mapped in a portable way by specifying <tt>@Transient</tt>. 
* The XML for extended mapping indicates which <tt>get()</tt> and <tt>set()</tt> method to use.

Example 2 

<source lang="java">
@Entity
  @VirtualAccessMethods(get="getExtension", set="setExtension")
  public class Address {
 
    @Id
    private int id;
 
    @Transient
    private Map<String, Object> extensions;
 
    public int getId(){
        return id;
    }
 
    public <T> T getExtension(String name) {
        return (T) extensions.get(name);
    }
 
    public Object setExtension(String name, Object value) {
        return extensions.put(name, value);
    }
 
...

</source>

<source lang="xml">
<basic name="name" attribute-type="String">
      <column name="FLEX_1"/>
      <access-methods get-method="getExtension" set-method="setExtension"/>
    </basic>
</source>

====Example 3====

Example 3  illustrates the following: 

* Property access is used for non-extension fields.
*Virtual access is used for extension fields, using defaults <tt>(get(String)</tt> and <tt>set(String, Object)) </tt> 
* The extensions are mapped in a portable way. No <tt>@Transient</tt> is required, because property access is used. 
*The <tt>get(String)</tt> and <tt>set(String, Object)</tt> methods will be woven, even if no mappings use them, because of the presence of <tt>@VirtualAccessMethods</tt>.<br>

<source lang="java">
@Entity
  @VirtualAccessMethods
  public class Address {
 
    private int id;
 
    private Map<String, Object> extensions;
 
    @Id
    public int getId(){
        return id;
    }
 
    public <T> T get(String name) {
        return (T) extensions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
 
...
</source>

<span id="this">
== Configuring the EntityManagerFactory and the Metadata Repository ==

'''''//REVIEWERS: I'm not sure about this section. Are these mostly implementation details whose user-facing information is already discussed above? Or should we retain this section to delve deeper into these subjects? If so, please advise what to say here.//'''''

* '''I think think this section can be fairly brief.  A quick intro to what the repository does and then an indication that it is possible to provide an overriding class and a pointer to the XML that provides the classname'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:30, 28 June 2011 (UTC)

Extensions are added at bootstrap time through access to a metadata repository. The metadata repository is accessed through a class that provides methods to retrieve the metadata it holds. 

Specify the class to use and any configuration information for the metadata repository through persistence unit properties. The entity manager factory checks the metadata repository while bootstrapping for additional mapping information. If additional mapping information is found, the entity manager factory integrates the into the metadata it uses to bootstrap. 

Two types of metadata repository are supported: XML and database.

* '''There is no "database" at the moment.  I'll fix the other doc'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:30, 28 June 2011 (UTC)

You can provide your own implementation of the class to access the metadata repository. 
Each metadata repository access class must specify an individual set of properties to use to connect to the repository.
'''''//REVIEWERS: Should we provide an example here?//'''''

* '''Brief example of how to use the XML.  You can use "User-Specified Example" below. And explain that the propeties that start with "com.foo" are implementer-defined.'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:30, 28 June 2011 (UTC)

<property name="eclipselink.metadata-source" value="myPackage.MyClass"/>

* '''And a note that says you can subclass either org.eclipse.persistence.internal.jpa.extensions.MetadataRepository or org.eclipse.persistence.internal.jpa.extensions.XMLMetadataRepository.''' --[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:30, 28 June 2011 (UTC)


===Examples ===
'''''//REVIEWERS: Can we say something more about these examples to explain them? What? And should all this information be moved up to [[#Configuring persistence.xml|Configuring persistence.xml]]?//'''''

* '''XML File examples is covered above and could possibly be removed here.'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:32, 28 June 2011 (UTC)

====XML File Example====
 
<source lang="xml">
<property name="eclipselink.metadata-source" value="XML"/>
<property name="eclipselink.metadata-source.xml.url" value="foo://bar"/>
</source>


====User-Specified Example====

<source lang="xml">
<property name="eclipselink.metadata-source" value="com.foo.MetadataRepository"/>
<property name="com.foo.MetadataRepository.location" value="foo://bar"/>
<property name="com.foo.MetadataRepository.extra-data" value="foo-bar"/>
</source>

{{EclipseLink_Note
|note=Use [[http://www.eclipse.org/eclipselink/api/latest/org/eclipse/persistence/jpa/JpaEntityManagerFactory.html#refreshMetadata(java.util.Map) RefreshMetadata()]] to refresh the metadata repository.
}}
''''''''//REVIERWERS: Should that note on refresh metadata be included? Should something more be said about it?//'''''
*'''refreshMetadata() is important.  We should indicate that if you change the metadata and you want an EntityManager based on the new metadata, you call refreshMetadata on the EntityManagerFactory and then the next EntityManager you get will be based on the new metadata.  Additionally, refreshMetadata takes a Map of properties and that map of properties can be used to override the properties previously defined for the metadata-source.'''--[[User:Tom.ware.oracle.com|Tom.ware.oracle.com]] 17:36, 28 June 2011 (UTC)


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