Validate contract number in Employment contracts business object.

Summary:

We need this validation on contract number to match a pattern. Also this should be only for Employees in Germany. Need expert help.

Content (required):

We need to validate contract number such that it matches a pattern. If it does not match , we need to pop up error. But my problem is to make it work only for Employee and not contingent worker. How to traverse and get assignment details from Employment contract. This rule should be valid only only for Assignment Type "E". So basically i need to know how to find the assignment type when we are creating rule for employment contracts.