The Action: Create New User
Controller - calls repository Create
in the Create method you edit if the user id is already in database. If it is you want to avoid an insert and returnthe error
ModelSate cannot be updated to signal the controller an error
once the validation is moved from the controller what is the best pattern to use.
in the above scenario a new Repository can be implemented 'DoesUserExist' to return the user name in the event the user id already exist.
This is a simple edit - but what if the validation is complex? How would one implement a robust validation process using the repository pattern