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As an alternative to Java code that must be compiled, you can use an interpreted expression language like OCL for coding constraints. The advantage is a shorter development cycle, the downside is slightly more complex configuration. And of course, there are both advantages and disadvantages to expression languages.  Adding and using such a Note that the available expression languages depend on what plugins are installed in Eclipse. E.g. OCL support is not part of the standard modelling package, so must be installed, but luckily it is available in the standard Eclipse update site, so it's easy to install.

Adding and using an interpreted constraint is done in three steps:

Step 1: Add an EAnnotation for using OCL for validation to the EPackage.

Besides installing the necessary OCL plugins, support Support for using OCL for validation must be explicitly indicated with an EAnnotation on the EPackage. Right-click on the EClass in the editor and add an EAnnotation. Set the source attribute to http://www.eclipse.org/emf/2002/Ecore, and add a key/value pair, with validationDelegate as the key and http://www.eclipse.org/emf/2002/Ecore/OCL as the value.

Step 2: Add an EAnnotation to the appropriate EClass.

This step is the same as step 1 in the previous section: Add an EAnnotation with the source attribute set to http://www.eclipse.org/emf/2002/Ecore anda key/value pair, with constraints as the key and the logical names of the constraints separated by a space.

Step 3: Add an EAnnotation for the constraint expression.

This step links the validationDelegate from step 1 to the constraint in step 2: Add another EAnnotation to the EClass, where the source attribute is http://www.eclipse.org/emf/2002/Ecore/OCL, i.e. the value for the validationDelegate key of step 1. Then add key/value pairs for each constraint, with the constraint name as the key and the OCL expression for the constraint as the value.

An example is shown in the figure to the right. The OCL expression is executed with an OrgUnit instance as self and computes the set of workers with the role of a manager and checks if its size is 1.