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Difference between revisions of "EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping/Relationship Mappings/Common Relationship Configurations/JoinFetch"

m (What You May Need to Know About EclipseLink JPA Lazy Loading)
m (How to Configure Default Fetch Group Behavior)
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  ReadAllQuery query2 = new ReadAllQuery(Employee.class);
 
  ReadAllQuery query2 = new ReadAllQuery(Employee.class);
 
  query2.setShouldUsedefaultFetchGroup(false);
 
  query2.setShouldUsedefaultFetchGroup(false);
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 +
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===How to Query with a Static Fetch Group===
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[[#Example 107-4|Configuring a Query with a FetchGroup Using the FetchGroupManager]] shows how to configure a <tt>ReadObjectQuery</tt> for the <tt>Employee</tt> class with a <tt>FetchGroup</tt> named <tt>nameOnly</tt> previously stored in the <tt>FetchGroupManager</tt> owned by the <tt>Employee</tt> class's descriptor.
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<span id="'Example 107-4"></span>
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'''' Configuring a Query with a FetchGroup Using the FetchGroupManager'''''
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In this example, only the <tt>Employee</tt> attributes <tt>firstName</tt> and <tt>lastName</tt> are fetched. If you call the <tt>Employee</tt> method <tt>get</tt> for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that <tt>get</tt> method will return the value directly from the object.
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'''// create static fetch group at the descriptor level'''
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FetchGroup group = new FetchGroup("nameOnly");
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group.addAttribute("firstName");
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group.addAttribute("lastName");
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descriptor.getFetchGroupManager().addFetchGroup(group);
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'''// use static fetch group at query level'''
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ReadAllQuery query = new ReadAllQuery(Employee.class);
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query.setFetchGroupName("nameOnly");
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===How to Query with a Dynamic Fetch Group===
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[[#Example 107-5|Configuring a Query with a FetchGroup Dynamically]] shows how to create a <tt>FetchGroup</tt> instance dynamically, at the time you create and execute a query, and configure the query with that <tt>FetchGroup</tt> directly.
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In this example, only the <tt>firstName</tt>, <tt>lastName</tt>, and <tt>salary</tt> attributes are fetched. If you call the <tt>Employee</tt> method <tt>get</tt> for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that <tt>get</tt> method will return the value directly from the object.
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 +
 +
<span id="Example 107-5"></span>
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''''' Configuring a Query with a FetchGroup Dynamically'''''
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 +
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'''// dynamic fetch group query'''
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ReadAllQuery query = new ReadAllQuery(Employee.class);
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FetchGroup group = new FetchGroup("nameAndSalary");
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group.addAttribute("firstName");
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group.addAttribute("lastName");
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group.addAttribute("salary");
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query. setFetchGroup(group);

Revision as of 11:28, 16 June 2010

Fetch Type

By default, EclipseLink persistence provider uses a fetch type of javax.persitence.FetchType.EAGER: this is a requirement on the persistence provider runtime that data must be eagerly fetched.If the default is inappropriate for your application or a particular persistent field, set fetch to FetchType.LAZY: this is a hint to the persistence provider that data should be fetched lazily when it is first accessed (if possible). We recommend using the FetchType.LAZY on all relationships.

You are not required to provide value for this attribute.

How to Configure Default Fetch Group Behavior

You can optionally designate at most one fetch group as the default fetch group for a descriptor's reference class.

If you execute a ReadObjectQuery or ReadAllQuery without specifying a fetch group, EclipseLink will use the default fetch group unless you configure the query otherwise, as this example shows.


Configuring Default Fetch Group Behavior

// at the descriptor level
FetchGroup group = new FetchGroup("nameOnly");
group.addAttribute("firstName");
group.addAttribute("lastName");
employeeDescriptor.getFetchGroupManager().addFetchGroup(group);
// set the default fetch group
employeeDescriptor.getFetchGroupManager().setDefaultFetchGroup(group);

// when query1 is executed, the default fetch group applies
ReadAllQuery query1 = new ReadAllQuery(Employee.class);

// when query2 is executed, the default fetch group does not apply
ReadAllQuery query2 = new ReadAllQuery(Employee.class);
query2.setShouldUsedefaultFetchGroup(false);


How to Query with a Static Fetch Group

Configuring a Query with a FetchGroup Using the FetchGroupManager shows how to configure a ReadObjectQuery for the Employee class with a FetchGroup named nameOnly previously stored in the FetchGroupManager owned by the Employee class's descriptor.


' Configuring a Query with a FetchGroup Using the FetchGroupManager

In this example, only the Employee attributes firstName and lastName are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.


// create static fetch group at the descriptor level
FetchGroup group = new FetchGroup("nameOnly");
group.addAttribute("firstName");
group.addAttribute("lastName");
descriptor.getFetchGroupManager().addFetchGroup(group);

// use static fetch group at query level
ReadAllQuery query = new ReadAllQuery(Employee.class);
query.setFetchGroupName("nameOnly");


How to Query with a Dynamic Fetch Group

Configuring a Query with a FetchGroup Dynamically shows how to create a FetchGroup instance dynamically, at the time you create and execute a query, and configure the query with that FetchGroup directly.

In this example, only the firstName, lastName, and salary attributes are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.


Configuring a Query with a FetchGroup Dynamically


// dynamic fetch group query
ReadAllQuery query = new ReadAllQuery(Employee.class);
FetchGroup group = new FetchGroup("nameAndSalary");
group.addAttribute("firstName");
group.addAttribute("lastName");
group.addAttribute("salary");
query. setFetchGroup(group);