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The "in a relationship" operand can be a whole relationship, a subclass of items in a relationship, or a product in a relationship. The "an expression" operand can be any rule template or any rule you construct from templates.
The purpose of the Relationship Item Constraint template is to allow you to write a rule for items in a relationship as if you had written the rule separately for each instance of the items. For example, you define Relationship A that contains the customizable product desktop PC. The desktop PC is a customizable product that includes two relationships: CPU and Hard Drive. You then write the following rule:
This means, for example, that if the user configures three desktop PCs, all with CPUs, then only one of them must have a hard drive. If the user removes the hard drive, the eConfigurator engine would add a hard drive to another desktop PC in the solution or add a new desktop PC that contains only a hard drive. The require rule defines a constraint that is true about the group of desktop PCs in the solution rather than about individual desktop PCs.
Another problem with the require rule is that it does not limit enforcement of the constraint to the items in Relationship A. If, in the require rule example, desktop PCs were also contained in Relationship B, then desktop PCs configured from Relationship B would also be considered when enforcing the require rule for desktop PCs configured from Relationship A.
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