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The Diagram View for Data Modeling

The diagram view for the data modeler contains the same sort of graphic elements as the class modeler, as illustrated in Figure 12-1. The semantics of the elements, however, are different:

• Nodes are entities, not classes.

• Compartments within a node show attributes and relationships.

• Lines represent relationships between entities.

Arrowheads on lines also have meaning. A single arrowhead denotes a to-one relationship; a double arrowhead denotes a to-many. The direction of an arrow indicates the direction of the relationship—the arrow points to the destination entity. Figure 12-1 shows an example of the diagram view of a data model, with all compartments expanded.

You can edit the model directly from the diagram using contextual menus. To add a new entity, you Control-click the background of the diagram. To add properties to an entity, you Control-click within its node. You can also delete entities and properties using the Delete key. Finally, you can use the line tool to establish new relationships. You select the line tool, then drag from the source node to the destination node.

Note that although relationships are typically implicitly bidirectional, relationships do not have to be modeled in both directions. If do you want to specify a bidirectional relationship, you must model both sides of the relationship—the reasons for this are given in the Core Data documentation. Moreover, within the model you must specify which relationships are the inverse of each other. To do this you need to use the model browser.

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