To specify how the relationships, roles, ports and objects are connected to each other press the Bindings button in the Graph Tool. A window opens showing how the components of the Graph type are bound together. The tool interface is similar to the Types tab, but it shows the bindings of the modeling language, each binding showing a possible way of connecting certain object types with role types and relationship types, possibly via ports.

A binding consists of a relationship type and a number of connections (usually two for a binary relationship). Each connection consists of a role type and a list of object types that may participate in the relationship in that role. The binding is defined by first adding a relationship type to the relationships list, selecting that relationship and adding role types for it, then selecting each of the role types and adding the object types that can take part in this relationship in that role.

If the use of ports is required, a suitable port instance or type is chosen between the role and object. Choosing a port instance requires that specific instance to be in the symbol of the object type; choosing a port type requires any instance of that type. In either case, when creating an instance binding in an editor, the user must attach the role to a matching port in the object.

If no bindings are defined, the default behavior is that no relationships can be created, as there is no way of knowing which role types should be associated with which relationship types. Thus we must define bindings that allow all the relationship-role-object type combinations we want, without allowing any combinations that should be illegal. Note that it is possible to define multiple bindings for each relationship type, and similarly role types and object types may participate in as many bindings as you wish, including being in multiple connections within the same binding. Types may also participate in several different graph types, with different bindings in each.

The members of a binding are selected from the available types currently in that graph type, defined in the Types tab (Section 2.5.3), plus their ancestors. In some exceptional circumstances it may be desirable to specify the supertype of all object types, Object: you can do this by selecting Add... with shift held down, and Object will then be included in the list of types to select from.

When creating a relationship in a Diagram Editor, the user can supply type information by choosing a specific type in the type toolbar. Normally, the type from the type toolbar (i.e. that defined in the Types tab) will be specified exactly in a binding in the Bindings tab, but it is also possible that the binding contains a type that inherits from the chosen type, or vice versa. As the selection in the Diagram Editor and the binding in the Graph type will both supply their own relationship type information, the more specific of the two will determine the type of the relationship instance that is created. Note that roles are always created to be instances of the exact type specified in the bindings.

In the Bindings tab the four lists are chained together, so that a binding is selected by selecting its relationship type in the relationship list. This then changes the contents of the other lists to show the connections in that binding: the role types and object types in them. We can then select a role type, and that will show us the object types that can participate in that role in that binding, along with their ports.

The order of the role types is important for each relationship type, because a user preference allows MetaEdit+’s tools to assume when drawing relationships that the object for the first role is always selected first, that for the second role is selected second etc. Hence you should try to select as the first role type in each binding the type which is most likely to be at the ‘starting’ end of the relationship. You can change the order of role types by dragging them within the list. Holding down Shift while dragging will create a new copy of the selected connection.

You can also choose Move... from the pop-up menu related to the role types:

The role list shows cardinalities for each connection in a binding. The cardinality is a range with minimum and maximum values and it defines how many times this connection with its role type and object types can appear in a single binding. An example of cardinality is the Child role in the Family relationship in Family Tree diagrams: as there can be zero to many children in a family, the cardinality for this role should be 0..N.

Cardinalities are set in the Cardinality Dialog, which is shown in Figure 2–9 below.

To set cardinalities:

The values can be from 0 to infinite, which is entered and shown as an N. If no cardinality is defined, the default is one for minimum and one for maximum, i.e. an obligatory role that can occur only once. If the upper limit is infinite set the maximum to N. You can of course also add the same role twice in the same binding; in general using cardinalities is a better way.