All relationships with the type must be configured explicitly.
Need to do this if you have more than one relationship with the same type. Examples might include a sporting Fixture, that has a relationship with the Team class both as a HomeTeam and an AwayTeam. Example here is for an EditorialEntry which has relationships denoting the AssignedIssue and the CommissionForIssue, both represented by the Issue type:
HasOptional(p => p.AssignedIssue).WithMany().HasForeignKey(p => p.AssignedIssueId);
HasOptional(p => p.CommissionedForIssue).WithMany().HasForeignKey(p => p.CommissionedForIssueId);
HasOptional becomes HasRequired when the relationship is required.