实体框架代码优先:与同一个表的一对多和多对多关系

Question

我的项目中有用户模型和事件模型.事件具有创建者(用户)并具有参与者(用户),因此事件与用户具有一对多关系,并且与同一个表具有多对多关系.

我首先得到了这样的一对多关系:

Public class Event
{
      ...
      public int CreatedById { get; set; }
      public virtual User CreatedBy { get; set; }
      ...
}

然后当我添加多对多关系时,迁移不会产生多对多的关系:

Public class User
{
      ...
      public virtual ICollection<Event> Events { get; set; }
      ...
}

Public class Event
{
      ...
      public int CreatedById { get; set; }
      public virtual User CreatedBy { get; set; }
      public virtual ICollection<User> Users { get; set; }
      ...
}

如果我删除了一对多关系,则迁移会成功生成多对多关系.

有没有办法只用数据注释来做到这一点？

Answer 1

EF不知道User.Events必须映射到哪里.它可能是Event.CreatedBy,也可能是Event.Users.两者都会产生有效的模型.您必须通过应用[InverseProperty]属性给EF一点提示:

public class User
{
    ...
    [InverseProperty("Users")]
    public virtual ICollection<Event> Events { get; set; }
    ...
}

Answer 2

使用Code First Approach,我总是建议使用流畅的API而不是使用DataAnnotations,它自动使用一些转换.

这样,您就会知道自己的确切配置.

如果我是你,这就是我要用的:

public class EventMap : EntityTypeConfiguration<Event>
{
    public EventMap()
    {
        this.HasRequired(m => m.CreatedBy) // envent must have a creator
            .WithMany() // a user can have 0,1 or more events created by him
            .HasForeignKey(m => m.CreatedById) // specify property to be used as FK
            .WillCascadeOnDelete(true); // delete all events created by user if that specific user is deleted

        this.HasMany(m=>m.Users) // an event can have 0,1 or more participants
            .WithMany(m=>m.Events) // a user can be a participant in 0,1 or more events                
            .Map(m => m.MapLeftKey("EventId").MapRightKey("UserId")); // this will generate intermediate table to hold participant information - dbo.EventUser with EventId & UserId
            // Cascade Delete is always true for Many to Many mapping. however, it doesn't delete entry in other table, it deletes entry in Joined Table only.
    }
}