之前有说过  Code First 重复外键   的一种解决方案。   http://blog.csdn.net/hanjun0612/article/details/50478134

虽然可以解决问题，不过我觉得配置起来非常麻烦，特别时多个表，多个外键的时候。

今天介绍一个简单的方案

数据库表结构：

采用 [InverseProperty("")]  特性来表明外键关系。

实体层：

U1:

public partial class U1

    {

        public U1()

        {

            this.Uother = new HashSet<Uother>();

            this.Uother1 = new HashSet<Uother>();

        }

        [Key]

        public int id { get; set; }

        public string name { get; set; }

        [InverseProperty("U11")]

        public virtual ICollection<Uother> Uother { get; set; }

        [InverseProperty("U21")]

        public virtual ICollection<Uother> Uother1 { get; set; }

    }

Uother:

public partial class Uother

    {

        [Key]

        public int id { get; set; }

        public Nullable<int> u1_id { get; set; }

        public Nullable<int> u2_id { get; set; }

        [ForeignKey("u1_id")]

        public virtual U1 U11 { get; set; }

        [ForeignKey("u2_id")]

        public virtual U1 U21 { get; set; }

    }

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这里来个复杂一点的，大家不用太关注，

U1

public class U

    {

        [Key]

        public int id { get; set; }

        public string name { get; set; }

    }

    public partial class U1:U

    {

        public U1()

        {

            this.U2 = new HashSet<U2>();

            this.U3 = new HashSet<U3>();

            this.U33 = new HashSet<U3>();

        }

        public Nullable<int> input_Qty { get; set; }

        public Nullable<int> output_Qty { get; set; }

        [InverseProperty("U1")]

        public virtual ICollection<U2> U2 { get; set; }

        [InverseProperty("U12")]

        public virtual ICollection<U2> U22 { get; set; }

        [InverseProperty("U1")]

        public virtual ICollection<U3> U3 { get; set; }

        [InverseProperty("U12")]

        public virtual ICollection<U3> U33 { get; set; }

    }

U2

public partial class U2

    {

        [Key]

        public int Id { get; set; }

        public Nullable<int> Uid { get; set; }

        public Nullable<int> Uid2 { get; set; }

        public string remark { get; set; }

        [ForeignKey("Uid")]

        public virtual U1 U1 { get; set; }

        [ForeignKey("Uid2")]

        public virtual U1 U12 { get; set; }

    }

U3

public partial class U3

    {

        [Key]

        public int Id { get; set; }

        public Nullable<int> Uid { get; set; }

        public Nullable<int> Uid2 { get; set; }

        public string remark3 { get; set; }

        [ForeignKey("Uid")]

        public virtual U1 U1 { get; set; }

        [ForeignKey("Uid2")]

        public virtual U1 U12 { get; set; }

    }

其实也就是如下关系，U2，U3表各有2个外键关联到U1。