title: Database design - ERD. Modeling Question
body: <p>Hopefully someone may be able to quickly advise the best way for me to model my ERD (I'm using IDEF1X &amp; ERWin not Chen Notation) although it doesn't really matter. </p>
  
  <p>Basically I have Three Entities </p>
  
  <ol>
  <li>Organisation</li>
  <li>Person</li>
  <li>Trustee</li>
  </ol>
  
  <p>Business rules state that a Trustee can be either a person or organisation. </p>
  
  <p>My question is, would it be correct to have two primary, foreign key relationships to Trustee. One from Organisation > Trustee and the second from Person > Trustee. Where one of the FK fields of Trustee will always be NULL depending on if the Trustee is a person or organisation. </p>
  
  <p>Or would it be better to have some sort of associative table between person and organisation (person_organisation) and create a PK, FK relationship from person_Organisation > Trustee. </p>
  
  <p>The second option seems better as there might be other entities that also need to link to both Organisation and person. </p>
  
  <p>Hopefully that makes sense. </p>
  
  <p>Many many thanks in advance</p>