Each derived property must have one or more expressions defining the criteria for searching the result elements.
It is strongly recommended that you specify the customization target in the customization element before defining the expression for its derived property.
To open the dialog for a new expression definition
Select the attribute with the stereotype «derivedPropertySpecification».
Open the attribute’s Specification window.
In the general specification pane (opened by default), click the Expression property and then click the ... button. The Expression dialog opens, and you can then construct predefined expressions or define your custom operation for calculating derived property values.
To learn more about specifying expressions, see the Specifying criteria for querying model page.