Entity Framework Core：如何將不同的對象實例映射到同一個實體？

Question

如何使用 Entity Framework Core 將實體的不同對象實例映射到同一行到數據表中？

換句話說，Entity Framework Core 如何比較尚未插入數據庫的實體（否則我猜它只需要比較主鍵）並確定兩個對象應該映射到單個實體？ 它是否比較實例引用？ 或者調用默認的 Equality 運算符？

總結我的問題，讓我們假設我有一所包含學生和老師的學校，並且我希望為每個老師姓名創建一個數據行，但我不能重用同一個老師實例，我怎么能告訴 EF 這樣做呢？

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

簡單的：

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();

這假設教師是在別處創建的，並與學生保存在同一工作單元中。

Answer 2

為什么要手動編碼 DbContext？ 請改用 Scaffold-DbContext。

1) 通過在 SQL Server 中添加新的“TeacherID”列來重新設計您的 Student 表

2）在SQL Server中創建Student表和Teacher表的關系

3) 運行 Scaffold-DbContext 來更新 DbContext 和實體

4）使用下面的代碼

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();