title: Username exists check in ViewModel/DomainModel?
body: <p>I have ViewModel for Insert/Update/Delete. DI done on controller with Ninject implementing respository patter.</p>
  
  <p>I am little confused. I have UserCreateViewModel bind to Create.aspx once submitted need to check UserName already in DB.</p>
  
  <p>For 1 and 2 I need Constructor DI to call IsUserRegistered method from respository.</p>
  
  <p>1- ViewModel.</p>
  
  <p>2- domain model.</p>
  
  <p>3- service layer. Is it right to add extra layer on to check UserName exists in DB?</p>
  
  <p>4- Someone suggested to use Attribute for that and DI in that.</p>