GTC and USER ID Generation Issue

I have seen one intresting issue so thought to share with you all:

Requirement:
DB/Flat File GTC is Trusted Reconciliation and we have to generate a unique User ID in OIM.


Assumption:
OIM Version: Oracle Identity Manager 10g


General Design:

1. As User ID is required field in OIM then we pass a Lietral Value say 'DUMMYID' in User ID field through GTC
2. We attach an Event Handler/Entity Adapter to generate Unique User ID and attach it on User Form
3. And this Event Handler will update the User ID field of OIM with generated Unique User ID
4. We make Reconciliation Rule on some other unique field of Trusted Source say "Employee Number"

Issue with the Design:

Reconciliation will work fine for the first time but when we run for the second time then it would be having 'DUMMYID' in User ID field and it will update the OIM User ID with 'DUMMYID' again.

And if we run reconciliation for more than one user then it will try to update same User ID i.e. 'DUMMYID' for all the users and will throw exception "Can not insert duplicate User Id"

Solution:

1. Don't pass 'DUMMYID' from GTC
2. Make User ID as Non Mandatory in OIM