7.9 数据注解特性--ForeignKey

外键特性，可以应用到类的属性中。Code-First默认的约定，对外键属性来说，假定外键属性的名称和主键属性是匹配的。

我们看一下，下面的代码：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EF2
{
    [Table("StudentMaster",Schema="WaHaHa")]
   public class Student
    {
        [Key]
        [Column(Order=)]
        public int StudentKey1 { get; set; }

        [Key]
        [Column(Order=)]
        public int StudentKey2 { get; set; }

        [MaxLength()]
        [ConcurrencyCheck]
        [Required]
        [Column(,TypeName="nvarchar")]
        public string StudentName { get; set; }

        [NotMapped()]
        public int? Age { get; set; }

        public int StdId { get; set; }

        [ForeignKey("StdId")]
        public virtual Standard Standard { get; set; }

    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;

namespace EF2
{
   public class Standard
    {
       [Key]
       public int StdId { get; set; }

       public string StandardName { get; set; }
    }
}

从上面的图中，可以看出，StdId在Student表中是外键，在Standard表中是主键。

这里是不是用类名+id的做法，使用外键。

默认的是使用这样的：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EF2
{
    [Table("StudentMaster",Schema="WaHaHa")]
   public class Student
    {
        [Key]
        [Column(Order=)]
        public int StudentKey1 { get; set; }

        [Key]
        [Column(Order=)]
        public int StudentKey2 { get; set; }

        [MaxLength()]
        [ConcurrencyCheck]
        [Required]
        [Column(,TypeName="nvarchar")]
        public string StudentName { get; set; }

        [NotMapped()]
        public int? Age { get; set; }

        public int StandardId { get; set; }   //类名+ID

        [ForeignKey("StandardId")]
        public virtual Standard Standard { get; set; }

    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;

namespace EF2
{
   public class Standard
    {
       [Key]
       public int StandardId { get; set; }     //类名+ID

       public string StandardName { get; set; }
    }
}

看看下面的：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EF2
{
    [Table("StudentMaster",Schema="WaHaHa")]
   public class Student
    {
        [Key]
        [Column(Order=)]
        public int StudentKey1 { get; set; }

        [Key]
        [Column(Order=)]
        public int StudentKey2 { get; set; }

        [MaxLength()]
        [ConcurrencyCheck]
        [Required]
        [Column(,TypeName="nvarchar")]
        public string StudentName { get; set; }

        [NotMapped()]
        public int? Age { get; set; }

        public int StandardRefId { get; set; }

        [ForeignKey("StandardRefId")]
        public virtual Standard Standard { get; set; }

    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;

namespace EF2
{
   public class Standard
    {
       [Key]
       public int StandardId { get; set; }

       public string StandardName { get; set; }
    }
}

看数据库：

这个意思就是，我们可以随便指定外键的名称，哈哈哈哈哈。。。