title: Create an optional foreign key using the fluid-API for Entity Framework 7
body: <p>I'm trying to create an optional foreign key using Entity Framework 7 and the Fluid-API. In EF v6.x we had the option to add this using <code>.WithOptional</code> or <code>.HasOptional</code>, but I cant find any equivalent functionality in EF 7.. any ideas?</p>
  
  <p>Br,
  Inx</p>