EF Code-First 学习之旅 一对多的关系

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public virtual Standard Standard { get; set; }
}

public class Standard
{
    public Standard()
    {
        Students = new List<Student>();
    }
    public int StandardId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}
   

上面的代码中，Student实体包含导航属性Standard，Standard实体包含集合导航属性Student，Code First的默认规则为1对多的关系

指定外键

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

        public int StdandardRefId { get; set; }

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

public class Standard
{
    public Standard()
    {
        Students = new List<Student>();
    }
    public int StandardId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

Fluent API配置

public class Student
{
    public Student(){ }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public int StandardId { get; set; }

    public virtual Standard Standard { get; set; }
}

public class Standard
{
    public Standard()
    {
        StudentsList = new List<Student>();
    }
    public int StandardId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        //one-to-many
        modelBuilder.Entity<Student>()
                    .HasRequired<Standard>(s => s.Standard) // Student entity requires Standard
                    .WithMany(s => s.Students); // Standard entity includes many Students entities

}

如果外键不符合默认规则

public class Student
{
    public Student(){ }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    //StdId is not following code first conventions name
    public int StdId { get; set; }

    public virtual Standard Standard { get; set; }
}

public class Standard
{
    public Standard()
    {
        StudentsList = new List<Student>();
    }
    public int StandardId { get; set; }
    public string Description { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

可以如下配置

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        //one-to-many
        modelBuilder.Entity<Student>()
                    .HasRequired<Standard>(s => s.Standard)
                    .WithMany(s => s.Students)
                    .HasForeignKey(s => s.StdId);

}

modelBuilder.Entity<Student>().HasRequired<Standard>(s => s.Standard)

表明Student必须包含Standard导航属性，

.WithMany(s => s.Students).HasForeignKey(s => s.StdId)表明Standard有多个Student，外键名为StdId。

另一种方法

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        //configure one-to-many
        modelBuilder.Entity<Standard>()
                    .HasMany<Student>(s => s.Students) Standard has many Students
                    .WithRequired(s => s.Standard)  Student require one Standard
                    .HasForeignKey(s => s.StdId);Student includes specified foreignkey property name for Standard
}

外键可空的配置

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        //one-to-many
        modelBuilder.Entity<Student>()
                    .HasOptional<Standard>(s => s.Standard)
                    .WithMany(s => s.Students)
                    .HasForeignKey(s => s.StdId);

}
   

配置如发生如下异常

One or more validation errors were detected during model generation:

Domain.Student_Standard: : Multiplicity conflicts with the referential constraint in Role 'Student_Standard_Target' in relationship 'Student_Standard'. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be '1'.

则需要配置外键为nullable类型