如何告诉 Entity Framework 级联删除会起作用

Question

Entity Framework 发现可能存在竞争级联删除。但是，在使用中这是不可能的。我需要知道如何告诉实体框架这个。

此应用程序用于处理政治志愿者，这些志愿者可以在州级、县级和/或竞选级别提供帮助。

1. 每个州都有一系列县和活动。
2. 每个州、县和竞选活动都有一系列事件。
3. 每个事件都有一个父/所有者，它是州、县或竞选活动。对于任何给定的事件，只设置其中一个。

我看到两个问题（也许还有更多）。

1. 如果你从事件追踪到它的三个所有者，那么这三个所有者都有一个事件集合。
2. 如果你从州追踪，它有一个事件集合，它还有一个县和活动的集合，每个都有一个事件集合。

我如何告诉 Entity Framework 和 SQL Server 在操作中不会有两个所有者都试图级联删除同一个事件？

相关问题。在 EventConfiguration.Configure 中，我如何告诉 DB 对于三个外键强制执行，在任何给定记录中，其中两个外键必须为空？

我运行 Update-Database 的错误是：

Microsoft.Data.SqlClient.SqlException (0x80131904): Introducing FOREIGN KEY constraint 'FK_Events_Counties_CountyId' on table 'Events' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

解决方案在此处作为 zip 提供。 或者您可以查看以下课程：

public class Campaign
{
    public int Id { get; private set; }
    public string Name { get; private set; }
    public required State State { get; set; }
    public ICollection<Event> Events { get; set; }
    public ICollection<User> Followers { get; set; }
    protected Campaign(int id, string name, DateTime created)
    {
        Id = id;
        Events = default!;
        Followers = default!;
    }
    public Campaign()
    {
        Events = new List<Event>();
        Followers = new List<User>();
    }
}

public class CampaignConfiguration : IEntityTypeConfiguration<Campaign>
{
    public void Configure(EntityTypeBuilder<Campaign> builder)
    {
        builder.HasIndex(new[] { "Name", "StateId" }).IsUnique();
        builder.HasIndex("StateId");
        builder.HasMany(x => x.Events)
            .WithOne(x => x.Campaign).OnDelete(DeleteBehavior.Cascade);
    }
}


public class County
{
    public int Id { get; private set;  }
    public string Name { get; private set; }
    public required State State { get; set; }
    public ICollection<Event> Events { get; set; }
    public ICollection<User> Followers { get; set; }
    protected County(int id, string name)
    {
        Id = id;
        Events = default!;
        Followers = default!;
    }
    public County()
    {
        Events = new List<Event>();
        Followers = new List<User>();
    }
}

public class CountyConfiguration : IEntityTypeConfiguration<County>
{
    public void Configure(EntityTypeBuilder<County> builder)
    {
        builder.HasIndex(new[]{"Name", "StateId"}).IsUnique();
        builder.HasMany(x => x.Events)
            .WithOne(x => x.County).OnDelete(DeleteBehavior.Cascade);
    }
}


public class Event
{
    public int Id { get; private set; }
    public required User Owner { get; set; }
    public Campaign? Campaign { get; set; }
    public County? County { get; set; }
    public State? State { get; set; }
    protected Event(int id, DateTime created)
    {
        Id = id;
    }
    public Event()
    {
    }
}

public class EventConfiguration : IEntityTypeConfiguration<Event>
{
    public void Configure(EntityTypeBuilder<Event> builder)
    {
        // fails!
        // builder.HasIndex(e => new[] { "CampaignId", "CountyId", "StateId" }).IsUnique();
    }
}


public class State
{
    public int Id { get; private set; }
    public string Name { get; private set; }
    public ICollection<County> Counties { get; set; }
    public ICollection<Campaign> Campaigns { get; set; }
    public ICollection<Event> Events { get; set; }
    public ICollection<User> Followers { get; set; }
    protected State(int id, string name)
    {
        Id = id;
        Counties = default!;
        Campaigns = default!;
        Events = default!;
        Followers = default!;
    }
    public State()
    {
        Counties = new List<County>();
        Campaigns = new List<Campaign>();
        Events = new List<Event>();
        Followers = new List<User>();
    }
}

public class StateConfiguration : IEntityTypeConfiguration<State>
{
    public void Configure(EntityTypeBuilder<State> builder)
    {
        builder.HasMany(x => x.Counties)
            .WithOne(x => x.State).OnDelete(DeleteBehavior.Cascade);
        builder.HasMany(x => x.Campaigns)
            .WithOne(x => x.State).OnDelete(DeleteBehavior.Cascade);
        builder.HasMany(x => x.Events)
            .WithOne(x => x.State).OnDelete(DeleteBehavior.Cascade);
    }
}


public class User
{
    public int Id { get; private set; }
    public ICollection<Campaign> Campaigns { get; set; }
    public ICollection<County> Counties { get; set; }
    public ICollection<State> States { get; set; }
    public ICollection<Event> Events { get; set; }
    protected User(int id, string name, string email)
    {
        Id = id;
        Campaigns = default!;
        Counties = default!;
        States = default!;
        Events = default!;
    }
    public User()
    {
        Campaigns = new List<Campaign>();
        Counties = new List<County>();
        States = new List<State>();
        Events = new List<Event>();
    }
}

public class UserConfiguration : IEntityTypeConfiguration<User>
{
    public void Configure(EntityTypeBuilder<User> builder)
    {
    }
}