C#中如何使用EF Core的关系配置？如何定义外键？

<pre class="brush:php;toolbar:false;">public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Order> Orders { get; set; }
}

public class Order
{
    public int Id { get; set; }
    public int UserId { get; set; }  // 外键
    public User User { get; set; }   // 导航属性
}

<pre class="brush:php;toolbar:false;">protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Order>()
        .HasOne(o => o.User)           // Order 拥有一个 User
        .WithMany(u => u.Orders)       // User 有多个 Order
        .HasForeignKey(o => o.UserId); // 外键是 Order 的 UserId
}

<pre class="brush:php;toolbar:false;">public class Order
{
    public int Id { get; set; }

    [ForeignKey("User")]
    public int UserId { get; set; }

    public User User { get; set; }
}

<pre class="brush:php;toolbar:false;">public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Profile Profile { get; set; }
}

public class Profile
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public string Bio { get; set; }
    public User User { get; set; }
}

<pre class="brush:php;toolbar:false;">modelBuilder.Entity<User>()
    .HasOne(u => u.Profile)
    .WithOne(p => p.User)
    .HasForeignKey<Profile>(p => p.UserId);

<pre class="brush:php;toolbar:false;">public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Course> Courses { get; set; }
}

public class Course
{
    public int Id { get; set; }
    public string Title { get; set; }
    public ICollection<Student> Students { get; set; }
}

<pre class="brush:php;toolbar:false;">modelBuilder.Entity<Student>()
    .HasMany(s => s.Courses)
    .WithMany(c => c.Students);

<pre class="brush:php;toolbar:false;">modelBuilder.Entity<Order>()
    .HasOne(o => o.User)
    .WithMany(u => u.Orders)
    .HasForeignKey(o => o.UserId)
    .OnDelete(DeleteBehavior.Cascade); // 删除用户时，其订单也被删除

<pre class="brush:php;toolbar:false;">public int? UserId { get; set; } // 可选关系