View source for Training:Constraints

{{Edited|July|12|2025}}
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There are other ways to introduce business rules in the model than using state machines and guards. You can use '''Constraints'''. 

The model already has several implicit constraints from the cardinalities of the association ends. If you have cardinality 1..4 and you have zero objects in that relation – then you have a broken constraint.

===== Define Constraints =====
You may also define your own constraints:

[[File:Model car constraints.png|frameless|397x397px]]

You can choose if a broken constraint (a constraint that evaluates to false) should be treated as Information, Warning, or an Error to the user.

===== Delete Constraints =====
You can also define the constraint as being a delete constraint only:

[[File:Constraint wrappper.png|frameless|437x437px]]

This way, you have explained at the model level that the domain does not consider it acceptable to delete a Car-object as long as we have the deposit unless it is in state Scrapped.

The delete constraints will be checked when Deleted by MDriven. As a result, the Delete operator is executed on the class.

Other things that are checked when the Delete operator is run are the Business Delete Rules that exist on all association endpoints:

[[File:Business delete rule.png|frameless|438x438px]]

As modelers, we should decide the best rule for each association end. In this case, is it acceptable to delete a Brand if Cars are left in the AllCarsOfThisBrand association? No, I think not. I am setting it to “MustBeEmpty.”

The association is in the other direction on the other hand.

[[File:Business delete rule need not be empty.png|frameless|438x438px]]

I set that to “NeedNotBeEmptyNoWarning” – because deleting a car object is okay even if it has a brand.

===== Constraints Evaluation in OCL =====
If you want to evaluate constraints in OCL, use [[OCLOperators constraints]], for example.

===== Usage of Constraints =====
Constraints automatically show up in ViewModels - but you may opt them out on a per-ViewModel level:
[[File:2021-04-08 10h46 32.png|none|thumb|484x484px]]
You can also access Constraints and their "brokenness" via OCL Operators brokenConstraints and [[OCLOperators constraints|Constraints]]:
[[File:2021-04-08 10h48 43.png|none|thumb|454x454px]]'''<span style="color: #1a50ad">The MDriven Book</span>''' - '''Next Chapter:''' [[Training:The ViewModel|The ViewModel]]

'''See also:''' [[OCLOperators_constraints]] 
[[Category:UML]]
[[Category:Constraints]]
[[Category:UI]]
[[Category:Expressions]]
[[Category:The MDriven Book]]