java-hibernate-mappings.md

title: Java: Hibernate p2: mappings date: 2012-03-28 18:30:13 tags: java,java-hibernate

To create a mapping, a one to many in this case, we first need to edit the src/main/resources/Employee.hbm.xml file to state the mapping.

	<class name="model.Employee" table="employee">
		<id name="id" column="id" type="long">
			<generator class="native"></generator>
		</id>
		<property name="name" column="name" type="string"></property>
		    <set name="certificates" cascade="all">
			 <key column="employee_id"/>
			 <one-to-many class="model.Certificate"/>
		    </set>      
	</class>

	<class name="model.Certificate" table="certificate">
	      <id name="id" type="long" column="id">
		 <generator class="native"/>
	      </id>
	      <property name="name" column="cert_name" type="string"/>
	 </class>

Note the new set tag. This means we'll store the new classes in a java Set. We give it a name that will relate to the name in the Certificate class we'll make. cascade=all means that we'll persist the Certificates as the same time as Employee. We then set the key column in our to-be-created certificate table, and the fact it's going to be a one-to-many table using the class as model.Certificate. We finally create a new class tag that is the same as the one we created in the previous tutorial.

The Employee class we defined before only changes in that we create a new Set of the Certificate classes, and make the setters and getters.

	...
	private Set certificates;
	...
	public Set getCertificates() {
	    return certificates;
	}
	public void setCertificates(Set certificates) {
	    this.certificates = certificates;
	} 
	...   

Now here's the Certificate class:

	package model;

	public class Certificate {

		private long id = 1L;
		
		private String name;
		
		public Certificate() {
		}

		public Certificate(String fname) {
			name = fname;
		}

		public long getId() {
			return id;
		}

		public void setId(Long id) {
			this.id = id;
		}

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}
	}        

It's possible that you may need to implement compareTo() and hashCode() in some instances.

Now in the main class file, First create a Set with the certificates in, then you can add them to the Employee, and then save the employee.

	HashSet set = new HashSet();
	Certificate cert = new Certificate("some python shit");
	Certificate cert1 = new Certificate("some go shit");
	set.add(cert); set.add(cert1);

	...

	Employee employee = new Employee("name");
	employee.setCertificates(set);
	employeeID = (Long) session.save(employee);       

When you delete an employee, it will also delete its certificates.