我有一个Project，可以由该组织的Member拥有，也可以由ExternalResourcePerson拥有。这意味着每个FK MemberId或ExternalResourcePersonId可以为空，但不能同时为空。如何建立这种关系？

我刚刚开始在其中引入“外部资源人”一词。也许ExternalResourcePerson类应该有一个List<Project>而不是List<ExternalResourcePersonEngagement>？

这些是类：

public class Project
{
    public int Id { get; set; }

    // A Project can either have a Member as owner ...
    public int MemberId { get; set; }
    public Member ProjectOwner { get; set; }

    // ... or an ExternalResourcePerson
    public int ExternalResourcePersonId { get; set; }
    public ExternalResourcePerson ExternalProjectOwner { get; set; }
}

public class Member
{
    public int Id { get; set; }

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Phone { get; set; }
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string Zip { get; set; }

    public List<Project> Projects { get; set; }
}

public class ExternalResourcePerson
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Address { get; set; }
    public string Organisation { get; set; }
    public string Expertise { get; set; }
    public string Email { get; set; }
    public string Phone { get; set; }
    public string Notes { get; set; }
    public List<ExternalResourcePersonEngagement> Engagements { get; set; }
}

public class ExternalResourcePersonEngagement
{
    public int Id { get; set; }
    public int ProjectId { get; set; }
    public Project Project { get; set; }
    public int ExternalResourcePersonId { get; set; }
    public ExternalResourcePerson Person { get; set; }
}

我见过this question，但无法对我的案子应用任何答案。