Inheritance mapping

divums · **Joined:** Mon Jun 18, 2007 4:07 am **Posts:** 3

Hi. I have a simple class hierarchy with 3 classes which are the following:

Code:

public class Device
{
        private int id;
        private string macAddress;
}

public class Logger : Device
{
        private int reportinterval;

}

public class Sensor : Device
{
        private int sampleRate;
        private int measuringTime;
}

The Device class is just an "interface", it is just the parent class of the other two classes to consist the common fields. I will use it in a gridview for example but when I select a row I wanna get the Logger or Sensor object, so actually there won't be any Device object in my application.

I checked the NHibertnate's documentation and I chose the one-table-per-hierarchy implementation but my tutor said it is not good because it may not work and I couldn't convince him it will. So I implemented a following relational database:

Code:

CREATE TABLE `device` (
  `Id` int(11) NOT NULL auto_increment,
  `MacAddress` varchar(4) NOT NULL,
  PRIMARY KEY  (`Id`),
  UNIQUE KEY `U_Device` (`MacAddress`)
) ENGINE=InnoDB;

CREATE TABLE `logger` (
  `Id` int(11) NOT NULL auto_increment,
  `ReportInterval` int(11) default NULL,
  `DeviceId` int(11) default NULL,
  PRIMARY KEY  (`Id`),
  KEY `FK_Logger_Device` (`DeviceId`)
) ENGINE=InnoDB;

CREATE TABLE `sensor` (
  `Id` int(11) NOT NULL auto_increment,
  `SampleRate` int(11) default NULL,
  `MeasuringTime` int(11) default NULL,
  `DeviceId` int(11) default NULL,
  PRIMARY KEY  (`Id`),
  KEY `FK_Sensor_Device` (`DeviceId`)
) ENGINE=InnoDB;

ALTER TABLE `logger`
  ADD FOREIGN KEY (`DeviceId`) REFERENCES `device` (`Id`) ON DELETE CASCADE ON UPDATE CASCADE;

ALTER TABLE `sensor`
  ADD FOREIGN KEY (`DeviceId`) REFERENCES `device` (`Id`) ON DELETE CASCADE ON UPDATE CASCADE;

My question is how to map this database into my object model, bacause I couldn't find any example yet. Could anyone help me? Thanx a lot.

xasp · **Posted:** Tue Aug 28, 2007 7:39 am

Hello,

This looks like a table-per-subclass model.
You'd have 3 classes: Device, Logger and Sensor. Logger and Sensor inherit from Device.
In your mapping file, you'd create a mapping file for Device and in that mapping file, use 'joined-subclass' tags to map the Logger and Sensor classes (the 'key column' tag should be set to DeviceId).

I think this should work.

marcal · **Posted:** Tue Aug 28, 2007 10:34 am

divums wrote:

I checked the NHibertnate's documentation and I chose the one-table-per-hierarchy implementation but my tutor said it is not good because it may not work and I couldn't convince him it will.

Is there some information we're missing here? Because if you are in the position of being able to define your database, table-per-hierarchy is usually the best option, and is certainly so for this hierarchy. The extra joins will almost certainly be more expensive than the discriminator column.

A couple of other points: you do not need an independent ID column in you subclass tables, DeviceID will suffice because there is an effective one-to-one relation with the ID on the device table. Also, if the Device class won't be used by itself, define it as an interface or abstract class--the inheritance mapping will still function.