一个表怎么可能有两个可以为空的 FK，而其中只有一个有值？

Question

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

我刚刚开始在其中引入“外部资源人员”一词。 也许ExternalResourcePerson -class 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; }
}

我已经看到了这个问题，但我无法将任何答案应用于我的案例。

Answer 1

只需将它们声明为可为空并添加负责正确分配它的方法。

一些不是生产就绪解决方案的基本示例：

//Marker
public interface IProjectOwner
{

}

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

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

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

    public void SetProjectOwner(IProjectOwner projectOwner)
    {    
        //Add some validation for this. Like checking if the project owner is set.
        if(projectOwner == null)
            throw new Exception("Project owner cannot be null.");

        if(projectOwner is Member )
        {
            ProjectOwner = projectOwner;
            MemberId = projectOwner.Id;
        }

        if(projectOwner is ExternalResourcePerson)
        {
            ExternalProjectOwner = projectOwner;
            ExternalResourcePersonId = projectOwner.Id;
        }
    }
}

public class Member : IProjectOwner
{
    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 : IProjectOwner
{
    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; }
}

您还可以添加一些方法来检索此人。