Entity Framework Core：如何将不同的对象实例映射到同一个实体？

Question

如何使用 Entity Framework Core 将实体的不同对象实例映射到同一行到数据表中？

换句话说，Entity Framework Core 如何比较尚未插入数据库的实体（否则我猜它只需要比较主键）并确定两个对象应该映射到单个实体？ 它是否比较实例引用？ 或者调用默认的 Equality 运算符？

总结我的问题，让我们假设我有一所包含学生和老师的学校，并且我希望为每个老师姓名创建一个数据行，但我不能重用同一个老师实例，我怎么能告诉 EF 这样做呢？

public class Student
{
    [Key]
    public int Id { get; set; }

    public string Name { get; set; }

    public Teacher Teacher { get; set; }
}

public class Teacher
{
    [Key]
    public int Id { get; set; }

    public string Name { get; set; }
}

public class SchoolContext : DbContext
{
    public DbSet<Teacher> Teachers { get; set; }
    public DbSet<Student> Students { get; set; }
}

public class SchoolGenerator
{
    public static void CreateASchool(SchoolContext context)
    {
        List<Student> students = new List<Student>()
        {
            new Student(){Name = "Student 1", Teacher = new Teacher(){Name = "Teacher 1"}},
            new Student(){Name = "Student 2", Teacher = new Teacher(){Name = "Teacher 1"}}, // I cannot re-use the previous instance of Teacher 1 for this student
            new Student(){Name = "Student 3", Teacher = new Teacher(){Name = "Teacher 2"}},
            new Student(){Name = "Student 4", Teacher = new Teacher(){Name = "Teacher 2"}},
        };

        context.Students.AddRange(students);
        context.SaveChanges(); // I want to create only TWO teacher rows, one by name, and FOUR student rows
    }
}

Answer 1

简单的：

var teacher1 = ...;
var teacher2 = ...;

List<Student> students = new List<Student>()
    {
        new Student(){Name = "Student 1", Teacher = teacher1 },
        new Student(){Name = "Student 2", Teacher = teacher1 },
        new Student(){Name = "Student 3", Teacher = teacher2 },
        new Student(){Name = "Student 4", Teacher = teacher2 },
    };

context.Students.AddRange(teacher1, teacher2);
context.Students.AddRange(students);

context.SaveChanges();

这假设教师是在别处创建的，并与学生保存在同一工作单元中。

Answer 2

为什么要手动编码 DbContext？ 请改用 Scaffold-DbContext。

1) 通过在 SQL Server 中添加新的“TeacherID”列来重新设计您的 Student 表

2）在SQL Server中创建Student表和Teacher表的关系

3) 运行 Scaffold-DbContext 来更新 DbContext 和实体

4）使用下面的代码

var teacher1 = new Teacher(){ Name = "Teacher 1" };
var teacher2 = new Teacher(){ Name = "Teacher 2" };

context.Teachers.AddRange(teacher1, teacher2);

List<Student> students = new List<Student>()
    {
        new Student(){Name = "Student 1", TeacherID = teacher1.ID },
        new Student(){Name = "Student 2", TeacherID = teacher1.ID },
        new Student(){Name = "Student 3", TeacherID = teacher2.ID },
        new Student(){Name = "Student 4", TeacherID = teacher2.ID }
    };

context.Students.AddRange(students);
context.SaveChanges();