how to generate barcode in vb.net 2008 Manipulating entities with EntityManager in Java

Maker Data Matrix 2d barcode in Java Manipulating entities with EntityManager

Manipulating entities with EntityManager
Data Matrix Generator In Java
Using Barcode creator for Java Control to generate, create ECC200 image in Java applications.
www.OnBarcode.com
Decoding ECC200 In Java
Using Barcode decoder for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
persistence provider is guaranteed to implement it. You should check your provider s documentation before spending too much time figuring out which entity columns should be lazily fetched. Loading related entities One of the most intricate uses of fetch modes is to control the retrieval of related entities. Not too surprisingly, the EntityManager s find method must retrieve all entities related to the one returned by the method. Let s take the ActionBazaar Item entity, an exceptionally good case because it has a many-to-one, a one-tomany, and two many-to-many relationships. The only relationship type not represented is one-to-one. The Item entity has a many-to-one relationship with the Seller entity (a seller can sell more than one item, but an item can be sold by only one seller), a one-to-many relationship with the Bid entity (more than one bid can be put on an item), and a many-to-many relationship with the Category entity (an item can belong to more than one category and a category contains multiple items). These relationships are depicted in figure 9.7. When the find method returns an instance of an Item, it also automatically retrieves the Seller, Bid, and Category entities associated with the instance and populates them into their respective Item entity properties. As we see in listing 9.8, the single Seller entity associated with the Item is populated into the seller property, the Bid entities associated with an Item are populated into the bids List, and the Category entities the Item is listed under are populated into the categories property. It might surprise you to know some of these relationships are retrieved lazily.
Code128 Encoder In Java
Using Barcode generation for Java Control to generate, create Code 128 image in Java applications.
www.OnBarcode.com
PDF-417 2d Barcode Maker In Java
Using Barcode drawer for Java Control to generate, create PDF417 image in Java applications.
www.OnBarcode.com
Figure 9.7 The Item entity is related to three other entities: Seller, Bid, and Category. The relationships to Item are many-to-one, one-to-many, and many-to-many, respectively.
QR-Code Creator In Java
Using Barcode printer for Java Control to generate, create Denso QR Bar Code image in Java applications.
www.OnBarcode.com
Code 128 Encoder In Java
Using Barcode printer for Java Control to generate, create Code 128A image in Java applications.
www.OnBarcode.com
Managing persistence operations
EAN / UCC - 14 Maker In Java
Using Barcode generation for Java Control to generate, create EAN 128 image in Java applications.
www.OnBarcode.com
British Royal Mail 4-State Customer Code Printer In Java
Using Barcode generator for Java Control to generate, create RM4SCC image in Java applications.
www.OnBarcode.com
All the relationship annotations we saw in chapter 8, including the @ManyToOne, @OneToMany, and @ManyToMany annotations, have a fetch element to control fetch modes just like the @Basic annotation discussed in the previous section. None of the relationship annotations in listing 9.8 specify the fetch element, so the default for each annotation takes effect.
Painting Data Matrix In Java
Using Barcode maker for BIRT Control to generate, create Data Matrix 2d barcode image in BIRT reports applications.
www.OnBarcode.com
DataMatrix Creator In None
Using Barcode maker for Office Excel Control to generate, create Data Matrix ECC200 image in Excel applications.
www.OnBarcode.com
Listing 9.8 Relationships in the Item entity
EAN 13 Creation In Java
Using Barcode creator for Android Control to generate, create EAN 13 image in Android applications.
www.OnBarcode.com
EAN 128 Creator In Java
Using Barcode generation for BIRT reports Control to generate, create GS1-128 image in BIRT reports applications.
www.OnBarcode.com
public class Item { @ManyToOne public Seller getSeller(){ ...
Scan Barcode In Visual Basic .NET
Using Barcode Control SDK for .NET Control to generate, create, read, scan barcode image in VS .NET applications.
www.OnBarcode.com
Creating UCC.EAN - 128 In None
Using Barcode generator for Software Control to generate, create EAN / UCC - 14 image in Software applications.
www.OnBarcode.com
Many-to-one with Seller
EAN / UCC - 13 Generation In VS .NET
Using Barcode drawer for .NET Control to generate, create UCC - 12 image in .NET applications.
www.OnBarcode.com
Code-39 Scanner In Visual Studio .NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
@OneToMany public List<Bid> getBids(){ ...
Paint Barcode In None
Using Barcode printer for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Paint PDF-417 2d Barcode In Visual Basic .NET
Using Barcode drawer for Visual Studio .NET Control to generate, create PDF-417 2d barcode image in .NET applications.
www.OnBarcode.com
One-to-many with Bids
EAN / UCC - 13 Generator In .NET
Using Barcode generation for ASP.NET Control to generate, create EAN / UCC - 14 image in ASP.NET applications.
www.OnBarcode.com
Code 128 Code Set C Reader In Visual Basic .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
@ManyToMany public List<Category> getCategories(){ ... }
Many-to-many with Categories
By default, some of the relationship types are retrieved lazily while some are loaded eagerly. We ll discuss why each default makes sense as we go through each relationship retrieval case for the Item entity. The Seller associated with an Item is retrieved eagerly, because the fetch mode for the @ManyToOne annotation is defaulted to EAGER. To understand why this make sense, it is helpful to understand how the EntityManager implements eager fetching. In effect, each eagerly fetched relationship turns into an additional JOIN tacked onto the basic SELECT statement to retrieve the entity. To see what we mean, let s see how the SELECT statement for an eagerly fetched Seller record related to an Item looks (listing 9.9).
Listing 9.9 SELECT statement for eagerly fetched Seller related to an Item
SELECT * FROM ITEMS Inner join for many-to-one INNER JOIN SELLERS ON ITEMS.SELLER_ID = SELLERS.SELLER_ID WHERE ITEMS.ITEM_ID = 100
Manipulating entities with EntityManager
As listing 9.9 shows, an eager fetch means that the most natural way of retrieving the Item entity would be through a single SELECT statement using a JOIN between the ITEMS and SELLERS tables. It is important to note the fact that the JOIN will result in a single row, containing columns from both the SELLERS and ITEMS tables. In terms of database performance, this is more efficient than issuing one SELECT to retrieve the Item and issuing a separate SELECT to retrieve the related Seller. This is exactly what would have happened in case of lazy fetching and the second SELECT for retrieving the Seller will be issued when the Item s seller property is first accessed. Pretty much the same thing applies to the @OneToOne annotation, so the default for it is also eager loading. More specifically, the JOIN to implement the relationship would result in a fairly efficient single combined row in all cases. Lazy vs. eager loading of related entities In contrast, the @OneToMany and @ManyToMany annotations are defaulted to lazy loading. The critical difference is that for both of these relationship types, more than one entity is matched to the retrieved entity. Think about Category entities related to a retrieved Item, for example. JOINs implementing eagerly loaded oneto-many and many-to-many relationships usually return more than one row. In particular, a row is returned for every related entity matched. The problem becomes particularly obvious when you consider what happens when multiple Item entities are retrieved at one time (for example, as the result of a JPQL query, discussed in the next chapter). (N1 + N2 + + Nx) rows would be returned, where Ni is the number of related Category entities for the ith Item record. For nontrivial numbers of N and i, the retrieved result set could be quite large, potentially causing significant database performance issues. This is why JPA makes the conservative assumption of defaulting to lazy loading for @OneToMany and @ManyToMany annotations. Table 9.4 lists the default fetch behavior for each type of relationship annotation.
Table 9.4 Behavior of loading of associated entity is different for each kind of association by default. We can change the loading behavior by specifying the fetch element with the relationship. Relationship Type One-to-one One-to-many Many-to-one Many-to-many Default Fetch Behavior EAGER LAZY EAGER LAZY Number of Entities Retrieved Single entity retrieved Collection of entities retrieved Single entity retrieved Collection of entities retrieved
Copyright © OnBarcode.com . All rights reserved.