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Tips for Specifying Relationships
The following tips are useful to keep in mind as you specify relationships in your model:
- The relationships you define in your model must reflect a corresponding implementation in the database, as well as the features supported by your adaptor. EOModeler doesn't know, for example, if a relationship is to-one or to-many, or if your adaptor supports left outer joins. You need to know your database and your adaptor, and specify relationships accordingly.
- Use the diagram view to quickly create pairs of inverse relationships by control-dragging between source and destination attributes.
- Use the Relationship Inspector to specify information about a relationship, such as whether it's to-one or to-many, its semantics (that is, the type of join represented by the relationship), and the name of the destination model (if the destination isn't in the current model).
- Relationships are created as to-one relationships. You need to change this setting if the two entities have a to-many relationship (for example, a movie has many roles).
- A relationship can be compound, meaning that it can consist of multiple pairs of connected attributes. You can specify additional pairs of attributes only in the Relationship Inspector. Simply select a second source attribute and a second destination attribute, and click Connect a second time.
- A to-one relationship from one primary key to another primary key must always have exactly one row in the destination entity-if this isn't guaranteed to be the case, use a to-many relationship. This rule doesn't apply to a foreign key to primary key relationship, where a NULL value for the foreign key in the source row indicates that no row exists in the destination.
- To-one relationships must join on the complete primary key of the destination entity as the join component.
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