Assertion Statements

It often makes sense to show relationship role names on a diagram, as well as, or instead of the relationship names. Figure 13.9 shows a simple example.

Figure 13.9 Relationship role names

On the ‘Cardinalities’ tab for a relationship, the entity role names and relationship cardinalities are combined into ‘assertion’ statements. These statements describe the relationship in plain English, and can be extracted in Reports and List Reports (select the Assertion property). As you change the cardinalities and role names in Figure 13.9, the depiction of the relationship changes, as do the ‘Assertion Statements’.

In Figure 13.9, the relationship has two role names, and PowerDesigner has placed them on the diagram in a way that makes assertion statements easier to read. You just read them in a clockwise fashion. Therefore, we have:

Ice Cream Flavor Theme – group – Ice Cream Flavor

and

Ice Cream Flavor – be grouped by – Ice Cream Flavor Theme

PowerDesigner combines these words with the relationship cardinality and optionality to create the assertion statements:

Each Ice Cream Flavor Theme must group one or more Ice Cream Flavors

Each Ice Cream Flavor may be grouped by at most one Ice Cream Flavor Theme.

There is a standard model extension[8] that converts entity names to the plural equivalent, which can be imported into any CDM or LDM. In the example above, this extension has been imported, and PowerDesigner has generated the plural name for the ‘Ice Cream Flavor’ entity. PowerDesigner creates the plural name by suffixing the entity name with ‘s’; this doesn’t work for some entities, so you also have the option of adding your own plural name via a new property in the entity’s ‘General’ tab. You can try this out in Exercise 11. See Chapter 23 to find out how to ensure this extension is available for all new data models.