Hibernate - O/R Mappings-Many-to-One Mappings - Hibernate

What are Hibernate Many-to-One mappings?

A many-to-one association is the most common kind of association where an Object can be associated with multiple objects. For example a same address object can be associated with multiple employee objects.

Define RDBMS Tables:

Consider a situation where we need to store our employee records in EMPLOYEE table which will have following structure:

Further, many employee can have same address, so this association can be presented using many-to-one association. We will store address related information in a separate table which has following structure:

Create both the RBDMS tables and keep them ready for the next implementation.

Define POJO Classes:

Let us implement a POJO class Employee which will be used to persist the objects related to EMPLOYEE table and having a variable of Address type.

Define Hibernate Mapping File:

Let us develop our mapping files which instructs Hibernate how to map the defined classes to the database tables. The <many-to-one> element will be used to define the rule to establish a many-to-one relationship between EMPLOYEE and ADDRESS entities.

You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml. You are already familiar with most of the mapping detail but let us see all the elements of mapping file once again:

  • The mapping document is an XML document having <hibernate-mapping> as the root element which contains two <class> elements corresponding to each class.
  • The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
  • The <meta> element is optional element and can be used to create the class description.
  • The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
  • The <generator> element within the id element is used to automatically generate the primary key values. Set the class attribute of the generator element is set to native to let hibernate pick up either identity, sequence or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
  • The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
  • The <many-to-one> element is used to set the relationship between EMPLOYEE and ADDRESS entities. The name attribute is set to the defined variable in the parent class, in our case it is address. The column attribute is used to set the column name in the parent table EMPLOYEE.

Finally, we will create our application class with the main() method to run the application. We will use this application to save few Employee's records alongwith their addresses and then we will apply CRUD operations on those records.

Compilation and Execution:

Here are the steps to compile and run the above mentioned application. Make sure you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.

  • Create hibernate.cfg.xml configuration file as explained in configuration chapter.
  • Create Employee.hbm.xml mapping file as shown above.
  • Create Employee.java source file as shown above and compile it.
  • Create Address.java source file as shown above and compile it.
  • Create ManageEmployee.java source file as shown above and compile it.
  • Execute ManageEmployee binary to run the program.

You would get following result on the screen, and same time records would be created in EMPLOYEE and ADDRESS tables.

If you check your EMPLOYEE and ADDRESS tables, they should have following records:

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