title: Association class attributes in Domain Model Class Diagram
body: <p>Hi, I have recently started to learn system analysis and design and am having some trouble understanding domain model class diagram (DMCD) association class. </p>
  
  <p>As per image, when drawing the DMCD, I'd like to understand if an association class is allowed to contain attributes of the classes it derives from. The Invoice needs to contain the attributes apptNo and svcName. </p>
  
  <p>Association class inquiry image:
  <img src="https://i.stack.imgur.com/sFN9D.jpg" alt="Association class inquiry"></p>
  
  <p>Do I include the attributes as shown in the image?
  Or do I assume that the Invoice would already have these attributes because it is derived from Appointment and Service and that it is not necessary to include them as they can be referred back to the keys apptNo and svcID?</p>
  
  <p>I am confused about the concept. How should I present the association class?
  Can someone please provide some guidance?</p>
  
  <p>Thank you.</p>