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<document> <header> <product>resin-ee</product> <title>Collections: The @OneToMany Relation</title> <description><p>The @OneToMany relation adds collection extensions to the query languageand provides a Java Collection containing the children. @OneToManyrepresents a collection of children belonging to a parent,like students in Gryffindor house at Hogwarts school.</p><p>The <a href="examples|amber-many2one/index.xtp">Many-To-One tutorial</a>illustrated that a many-to-one relation links one source entity to anothertarget entity. A one-to-many relation links the target entity back to thesource entity.</p><p>In this example, each House has many Students, each Student has one House.House has a one-to-many relationship with Student,Student has a many-to-one relationship with House</p> </description> <type>tutorial</type> <tutorial-startpage>one2many</tutorial-startpage> </header> <body> <summary/><s1 title="Files in this tutorial"><deftable><tr><td><viewfile-link file="WEB-INF/resin-web.xml"/> </td><td>web.xml configuration</td></tr><tr><td><viewfile-link file="WEB-INF/classes/META-INF/persistence.xml"/> </td><td>persistence.xml configuration</td></tr><tr><td><viewfile-link file="WEB-INF/classes/example/Student.java"/> </td><td>The student bean</td></tr><tr><td><viewfile-link file="WEB-INF/classes/example/House.java"/> </td><td>The house bean</td></tr><tr><td><viewfile-link file="WEB-INF/classes/example/OneToManyServlet.java"/> </td><td>The test servlet</td></tr></deftable></s1><s1 title="Data Model: Object and Database Models"><s2 title="Database Schema"><p>The database schema is unchanged from the<a href="../amber-many2one/index.xtp">Many-To-One tutorial</a>,and might look like:</p><example title="SQL">CREATE TABLE house ( id BIGINT PRIMARY KEY auto_increment, name VARCHAR(250),)CREATE TABLE student ( id BIGINT PRIMARY KEY auto_increment, name VARCHAR(250), house BIGINT REFERENCES house(id))</example></s2><s2 title="@OneToMany - the relation's "inverse" side "><p>The <var>House</var> bean has a students field that returns the students that belong to the house. House annotates <code>getStudents</code> with <code>@OneToMany</code> to describe the relationship. The <code>@JoinColumn</code> is used to specify the <i>foreign key</i>, the field in <var>Student</var> that is the <code>@ManyToOne</code> link.</p><example title="House.java"> @OneToMany(mappedBy="house") private Set<Student> _students;</example></s2><s2 title="@ManyToOne - the relation's "owning" side"><p>A @OneToMany always requires a corresponding @ManyToOne on the target entity.The Student has a house field annotated a <code>@ManyToOne</code>,unchanged from the <a href="../amber-many2one/index.xtp">Many-To-One tutorial</a>.</p><example title="Student.java"> @ManyToOne @JoinColumn(name="house") private House _house;</example></s2></s1><!--<section title="@FetchType"><p>Often a <code>@OneToMany</code> relationship should be marked as <var/lazy/>.This indicates that the target entities should not be retrieved from thedatabase until the <code>@OneToMany</code> field is used.</p><p>The <code>@OneToMany</code> annotation for the students field indicates aFetchType of <code/LAZY/>. A House can potentially have many Students, and inuse the students may not be needed by the code using the House bean. Forexample, if the goal is to display a summary list only Houses, the associatedStudent's will not be needed and it would be inefficient to fetch them eachtime.</p><example title="House.java"> @OneToMany(targetEntity=Student.class, mappedBy="house") public Set<Student> getStudents()</example></section>--><s1 title="java.util.Collection: Using House.getStudents()"><p>The one-to-many relation provides a House the ability to get all of itsStudents. Resin will perform any necessarydatabase lookup.</p><p>The example queries all houses and prints theirnames and all of their students.</p><example title="Using House.getStudents()">private void doService(PrintWriter out) throws java.io.IOException{ public void service(HttpServletRequest req, HttpServletResponse res) throws java.io.IOException, ServletException { PrintWriter out = res.getWriter(); res.setContentType("text/html"); String sql = "SELECT h FROM House h"; Query allHouse = _entityManager.createQuery("SELECT o FROM House o"); List houses = allHouse.getResultList(); for (int i = 0; i < houses.size(); i++) { House house = (House) houses.get(i); out.println("<h3>" + house.getName() + "</h3>"); for ( Student student : house.getStudents() ) { out.println( student.getName() + "<br>" ); } } }}</example></s1><s1 title="Query extensions"><s2 title="Result Collections: SELECT h.students"><example title="h.students">SELECT h.students FROM House h WHERE h.name='Gryffindor'</example></s2><s2 title="Joins: FROM House h, IN(h.students) s"><example title="IN">SELECT s FROM House h, IN(h.students) s WHERE h.name='Gryffindor'</example></s2><s2 title="Membership: s MEMBER OF h.students"><example title="MEMBER OF">SELECT s FROM Student s, House h WHERE s MEMBER OF h.students</example></s2><s2 title="Empty: h.students IS EMPTY"><example title="IS EMPTY">SELECT h FROM House h WHERE h.students IS EMPTY</example></s2></s1> </body></document>⌨️ 快捷键说明
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