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<document>  <header>    <product>resin-ee</product>    <title>Links: The @ManyToOne Relation</title>        <description><p>The Many-to-One link is the foundation of persistent relations.It links a source table to a destination with a database REFERENCES column.Many-to-One adds two capabilities: SQL extensions for links and direct lookup of target beans through field references.</p>        </description>    <type>tutorial</type>    <tutorial-startpage>many2one</tutorial-startpage>  </header>  <body>    <summary/><s1 title="Files in this tutorial"><deftable><tr><td><viewfile-link file="WEB-INF/resin-web.xml"/>    </td><td>resin-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/ManyToOneServlet.java"/>    </td><td>The test servlet</td></tr></deftable></s1><s1 title="Model: Database and Annotated Classes"><s2 title="Database Schema"><p>A many-to-one relation links one table to another.  In this example,each student entry links to the student's house.  The database schemamight 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><p>The House bean has two fields, <var>id</var> and <var>name</var>,to model the house columns.  House annotates <code>getId</code> with <code>@Id</code> to mark it as a primary key.  It annotates <code>getName</code>with <code>@Basic</code> to mark it as a data column.</p><example title="House.java"><a href="doc|amber-table.xtp#@Entity">@Entity</a>public class House {  <a href="doc|amber-table.xtp#@Id">@Id</a>  <a href="doc|amber-table.xtp#@Column">@Column</a>(name="id")  public long getId()  <a href="doc|amber-table.xtp#@Basic">@Basic</a>  public String getName()}</example><p>The Student bean adds a many-to-one field, <var>house</var> to the<var>id</var> and <var>name</var> columns.  It marks the <code>getHouse</code>getter with <code>@ManyToOne</code> to mark it as a many-to-one column.</p><example title="Student.java"><a href="doc|amber-table.xtp#@Entity">@Entity</a>public class Student {  <a href="doc|amber-table.xtp#@Id">@Id</a>  <a href="doc|amber-table.xtp#@Column">@Column</a>(name="id")  public long getId()  <a href="doc|amber-table.xtp#@Basic">@Basic</a>  public String getName()  <a href="doc|amber-table.xtp#@ManyToOne">@ManyToOne</a>  <a href="doc|amber-table.xtp#@JoinColumn">@JoinColumn</a>(name="house")  public House getHouse()}</example><s2 title="@ManyToOne"><p><a href="doc|amber-table.xtp#@ManyToOne">@ManyToOne</a> marks thefield as a many-to-one field.  The <code>@ManyToOne</code> annotationcan also specify the target bean.  By default, the field's type determinesthe target bean.</p></s2><s2 title="@JoinColumn"><p><a href="doc|amber-table.xtp#@JoinColumn">@JoinColumn</a> specifiesthe SQL column name and the target column for a many-to-one field.</p><p>The default column name is the name of the many-to-one field.</p><p>The default target column is the primary key of the target table.</p></s2></s1><s1 title="Client Servlet"><p>The many-to-one relation provides two capabilities to the client:</p><ol><li>Extended SQL relation capabilities</li><li>A direct lookup to a persistent bean through the field.</li></ol><s2 title="Query Extensions"><p>The first relation capability extends SQL with relation pathsusing the '.' operator like Java's field reference.If <code>s</code> is a student,then <code>s.house</code> is the student's houseand <code>s.house.name</code> is the name ofthe student's house.  Resin translates the extended SQL to plainSQL automatically.</p><p>The following example uses the extended SQL to return all studentsin a given house.</p><example>private void doService(PrintWriter out)  throws java.io.IOException{  String sql = "SELECT s FROM Student s WHERE s.house.name=?1";      Query houseStudents = _entityManager.createQuery(sql);  houseStudents.setParameter(1, "Gryffindor");  students = houseStudent.getResultList();  out.println("&lt;h3&gt;Gryffindor Students&lt;/h3&gt;");  for (int i = 0; i &lt; students.size(); i++) {    Student student = (Student) students.get(i);    out.println(student.getName() + "&lt;br&gt;");  }}</example></s2><s2 title="Java References"><p>The second relation capability returns a live, persistent bean throughthe <code>getHouse()</code> method.  Amber will perform any necessarydatabase lookup.</p><p>The example queries all students and prints theirnames and house names.</p><example title="ManyToOneServlet.java">private void doService(PrintWriter out)  throws java.io.IOException{  Query allStudent = _entityManager.createQuery("SELECT o FROM Student o");      List students = allStudent.getResultList();  for (int i = 0; i &lt; students.size(); i++) {    Student student = (Student) students.get(i);    out.println(student.getName() + " lives in " +                student.getHouse().getName() + "&lt;br&gt;");  }}</example></s2></s1>  </body></document>
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