title: Persisting UI Related info to the Database in a Layered App
body: <p>my team and I are refactoring our application into a layered application. The app is a windows app that is organized with a UI, Service Layer(SL), Business Layer(BLL) and a Data Access Layer(DAL).  Our goal is to keep the Application Logic in the Service Layer, keep the Domain Logic in the Business Layer and communicate between the Service Layer and the UI with DTOs.  With that said, my question relates to UI specific data that is stored in the database.  </p>
  
  <p>For example  (A screen that has 5 phone numbers where the user gets to select the position{1,2,3,4 or 5} that those phone numbers show up on the screen).  In my Domain model, I have an entitiy called Phone</p>
  
  <pre><code>public class phone{
   string name;
   string number;
   string extension;
   etc...
  }
  </code></pre>
  
  <p>In the Service Layer I have a DTO name phoneDTO that looks like this</p>
  
  <pre><code>public class phoneDTO{
       string name;
       string number;
       string extension;
       int position;
       etc...
  }
  </code></pre>
  
  <p>Typically, i pass the DTO to the service layer and let the service layer create the objects from the domain model and then call the appropriate DAL mappers.</p>
  
  <p>Question:  When persisting the UI related info, what's the best practice for doing so? In my situation the DAL does not have a reference to the DTOs, It does have a reference to the Domain Model.  </p>