Currently our Graph Tool’s Types page looks like Figure 3-18, with all the required metamodel fragments for the first version of our Family Tree modeling language in place: the Person object, Family relationship, and Parent and Child roles.

What we need to do now is to specify the rules for how modelers can connect instances of these types, by defining the bindings. A binding contains the information on which objects can take part in a relationship in which roles, and how many times each role may occur. Each binding consists of one relationship, two or more roles, and for each role, one or more objects. A binding thus starts from a relationship and says which roles it may have, and then for each role, which objects may be in that role. You can have more than one binding for a relationship type; together, they list all the legal ways of connecting.

Go to the Bindings page in the Graph Tool. Here you will create the bindings that define how objects can be connected with relationships and roles. In the Relationships list, press the right mouse button, select Add... from the pop-up menu and choose ‘Family’ from the list.

Make sure that ‘Family’ is selected in the Relationships list. Go to the Roles list and press the right mouse button there. Choose Add... and select ‘Parent’ from the list. Now, with both ‘Family’ and ‘Parent’ selected, add ‘Person’ to the Objects list (skip the Ports list). You have now created a part of the binding that says: “A Family relationship has a Parent role that connects to a Person object”.

As there are always two Parents for a Person, proceed by keeping the same Family relationship still selected and add another Parent role to the Roles list and a Person for it in the Objects list. The Graph Tool should now look like Figure 3-19.

In addition to the Parent roles, there is also a Child role in the Family relationship. To create it, move the mouse over the Roles list, press the right mouse button, choose Add... from the pop-up menu and then ‘Child’ from the list of available roles. As the Objects list for Child is now blank, go there and add the Person object again. We have now created the part of a binding that says: “The Family relationship has a Child role that connects to a Person object”.

As the problem domain specification and our metamodel implies, there are certain constraints that limit the set of possible Family combinations: there can be two and only two Parents but zero to many Children in a Family. In GOPPRR, these constraints are specified in the bindings. The requirement for two Parents is already taken care of with two separate Parent roles, but the cardinality of ‘zero to many Children’ must still be set. Select the Child role from the Roles list, press the right mouse button and choose Cardinality... from the pop-up menu. In the Cardinality Dialog that appears (Figure 3-20), set the Minimum to 0 and the Maximum to N and press OK.

Your Graph Tool should now look like Figure 3-21.

Press Save and Close to save the graph definition and close the Graph Tool. Also remember to Commit your work again.

Congratulations! You have now completed the first version of our Family Tree modeling language.