different models for one table - symfony

Question

Is it possible (and recommended) to use multiple models for one table?
I have multiple types of "people" (teacher, students, employees, firms) and would like to store them in the same table. They have all personal data in common, but have other relations (for ex. students -> firms, teacher -> rooms and more), additional information (for ex. firms: firm name, teachers: education) and more.
It would be nice to get some best practices for such cases.

Answer 1

It's really simple! You have to make entities like this:
/**
* #ORM\Entity
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="discr", type="string")
* #ORM\DiscriminatorMap({"teacher" = "Teacher"})
*/
class Person
{
/**
* #ORM/Id
**/
private $id;
/**
* #ORM/Column
**/
private $name;
//setters getters and all the stuff
}
And in Teacher.php file:
/**
* #ORM\Table()
* #ORM\Entity()
*/
class Teacher extends Person
{
private $salary;
}
The key to success are those two annotation:
#ORM\DiscriminatorColumn(name="discr", type="string")
#ORM\DiscriminatorMap({"teacher" = "Teacher"})
First is telling ORM what column will be used to verify if person is Teacher or not.
Second is telling what classes are extending base Person class and what to put in column i mentioned before. When you do that you will have two tables, but in Teacher you will have only data you add to Teacher entity :)
Generally speaking when using ORM you have to think on the object abstraction level and don't care about DB (well, that's not entirely true, but it's general idea) :)

Answer 2

It's not recommended. Here are several reasons I can think of why not to do this:
If Teachers and Students share the same set of IDs, what is to stop someone from loading a Teacher using a Student's ID, and adding a value to the "salary" field? Limiting this access can be done, but it may require custom repositories or other modifications.
Let's say that Teachers are required to have a salary. If Students and Teachers share the same table, then the database won't be able to enforce that constraint without giving Students a salary too.
Even though Students will never have a salary, the database may need to allocate space for that field anyway.
IBM has a great post about mapping inheritance in relational databases, and I've referred to it several times when implementing this type of model. They make reference to three methods: (1) using a single table to represent all classes (the method you've proposed), (2) using separate tables for children classes, or (3) using separate tables for all classes. In the end, it comes down to personal preference, but the way I normally do it is a blend between #2 and #3, where I create the model for all classes, but I limit the access to the parent class from the child, and instead write shortcut methods to access the parent data. Consider this:
class Person
{
private $id;
private $name;
public function getId()
{ return $this->id; }
public function getName()
{ return $this->name; }
public function setName($name)
{ return $this->name; }
}
class Teacher
{
private $id;
private $person; // reference to Person table
private $salary;
public function getId()
{ return $this->id; }
private function getPerson()
{ return $this->person; }
public function getSalary()
{ return $this->salary; }
public function setSalary($salary)
{ $this->salary = $salary; }
public function getName()
{ return $this->getPerson()->getName(); }
public function setName($name)
{ $this->getPerson()->setName($name); }
}
I usually choose this method because I can treat a Teacher as simply a Teacher, or as a Person, depending on what the situation requires. So let's say that I have two pages: one that prints out everyone's name (Teachers and Students), and one that prints out just the Teachers and their salaries:
// Assume:
// $people = $this->getDoctrine()->getEntityManager()->getRepository("MyBundle:Person")->findAll()
{% for person in people %}
{{ person.name }}
{% endfor %}
// Assume:
// $teachers = $this->getDoctrine()->getEntityManager()->getRepository("MyBundle:Teacher")->findAll()
{% for teacher in teachers %}
{{ teacher.name }} makes ${{ teacher.salary }}
{% endfor %}