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Using many-to-one relationships to join tables Building the database with Ant and SchemaExport Using components to make finely grained object models
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p until now, you have seen simple queries that basically pull data from a single table. The additional work of mapping a single persistent object might not seem worth the trouble. The real value of using an ORM framework like Hibernate is that you can connect objects together and then fetch an entire object graph with a simple query. Take a seemingly insignificantly small query like the following:
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List list = session.find("from Event");
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This query could return 1, 10, or 1000 persistent objects from the database, including not only Events but other objects linked to each Event. This approach is extremely efficient if you need all of them, and Hibernate even allows you to expand or shrink the scope of which objects are pulled from the database. One of the ways to do this is to define associations between persistent objects.
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Nearly any relationship between two objects you can write can be mapped to a relational database by Hibernate. Powerful stuff indeed. This chapter covers how you can build those rich object models and turn over the heavy lifting to Hibernate to convert them back and forth between Java and the database.
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This chapter is all about relationships and rich object models. We expand our sample application a bit, and along the way we explore how Hibernate can bring objects together. You ll accomplish the following:
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Create a unidirectional many-to-one association between the Event and Locations. Automatically build a database table from our mapping documents using SchemaExport and Ant. Use a component to create an Address object, a finely grained object that doesn t get its own table, as entities do.
Assumptions
This chapter builds on what you have learned in previous chapters, so we assume you should be able to do the following:
Configure a Hibernate SessionFactory using the hibernate.cfg.xml file. Make a single object persistent using a Hibernate mapping document. Obtain a session from the SessionFactory and use it to persist and load objects.
4.1 Associations
The simplest association that Hibernate supports is linking two entities together. Entity is a term for an object that has its own persistent
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identity.1 For example, the Events you have worked with so far are entities. Even if two events had the same name and date, they might be completely different events, differing only by their identity. In your applications, Locations are also entities; each one has a unique identity. After all, there is probably an Oak Street in every suburb in America, but each one is a different street.2 In our application, every Event is held at a single Location only. The way you would represent this in Java is to have an Event object with a Location field. When you retrieve an Event, you usually want the Location too. So you are going to link Event and Location together using a many-to-one relationship.
4.1.1 Many-to-one relationships, in depth
In section 3.2, you saw a sample mapping file for an Event. Here we go a little deeper and explore a many-to-one relationship in a bit more depth.
Defining the Event and Location classes
In this section you ll create an Event class, with a many-to-one relationship to Location. From a detailed UML perspective, figure 4.1 shows what this relationship will look like. In other words, many events can be in single location. First, create two classes (shown in listing 4.1 and listing 4.2), in the /work/calendar/src/ java/com/manning/hq/ch04 directory.
Event -id : long -name : string -duration : int -startDate : Date -location Location -id : long -name : string -address : string
Figure 4.1 UML diagram of Event and Location
The generic term entity is not to be confused with entity beans, which EJB uses to make objects unique. Old joke: Suburbs are where they cut down trees and then rename the streets after them.
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