title: Is it possible to create a Vertex that requires an edge in order to be created
body: <p>I'd like to know if it is possible to create a Vertex that requires an edge in order to be created. </p>
  
  <p>For example, I want to create an <code>Invoice</code> class that has a <code>HasCustomer</code> edge which points to Person. </p>
  
  <p>I want the <code>HasCustomer</code> edge to be mandatory in order to create the Invoice. </p>
  
  <p>You cannot create the <code>Invoice</code> unless you have the <code>HasCustomer</code> edge.</p>
  
  <p>I know we can have a <code>link to Person</code>, but there is no referential integrity. I can delete the <code>Person and the Invoice</code> will simply end up with a link to a customer that is non-existent.</p>