Difference between revisions of "Epsilon/EMF"

Revision as of 16:11, 14 December 2014

What is the difference between containment and non-containment references in EMF?

Briefly, a model element can belong to as most one containment reference at a time. Containment references also demonstrate a cascade-delete behaviour. For example, consider the following Ecore metamodel (captured in Emfatic).

 package cars;
 
 class Person {
   ref Person[*] friends; //non-containment reference
   val Car[*] cars; // containment reference
 }
 
 class Car { }

Now consider the following EOL code which demonstrates the similarities/differences of containment and non-containment references.

 // Set up a few model elements to play with
 var c1 = new Car;
 var c2 = new Car;
 var p1 = new Person;
 var p2 = new Person;
 var p3 = new Person;
 
 // p1's car is c1 and p2's car is c2
 p1.cars.add(c1);
 p2.cars.add(c2);
 
 // p3 is a friend of both p1 and p2
 p1.friends.add(p3);
 p2.friends.add(p3);
 
 p1.friends.println(); // prints {p3}
 p2.friends.println(); // prints {p3}
 
 //add c2 to p1's cars
 p1.cars.add(c2);
 p1.cars.println(); // prints {c1, c2}
 
 // The following statement prints an empty set! 
 // As discussed above, model elements can belong to at 
 // most 1 containment reference. As such, by adding c2 to
 // the cars of p1, EMF removes it from the cars of p2
 p2.cars.println();
 
 // Delete p1 from the model
 delete p1;
 
 Person.all.println(); // prints {p2, p3}
 
 // The following statement prints an empty set!
 // As discussed above, containment references demonstrate
 // a cascade-delete behaviour. As such, when we deleted p1,
 // all the model elements contained in its cars containment reference
 // were also deleted from the model. Note how the friends of p1 (p2 and p3)
 // were not deleted from the model, since they were referenced through a 
 // non-containment reference (friends)
 Car.all.println();