举例说明EF CORE中模型之间的一对多、多对多关系的实现

该例子是我临时想出来的，不具有任何的实际意义。类图如图1所示。

图1

类代码：

[Table("student")]
    public class Student
    {
        public int id { get; set; }
        public string name { get; set; }
        public int grade_id { get; set; }
        public int address_id { get; set; }

        [NotMapped]
        public Address address { get; set; }

        [NotMapped]
        public List<Subject> subjects { get; set; }
        [NotMapped]
        public List<Teacher> teachers { get; set; }

    }

[Table("teacher")]
    public class Teacher
    {
        public int id { get; set; }
        public string name { get; set; }
        public int subject_id { get; set; }
        public int depart_id { get; set; }
        public int address_id { get; set; }

        [NotMapped]
        public Address address { get; set; }

        [NotMapped]
        public List<Student> students { get; set; }

    }

[Table("grade")]
    public class Grade
    {
        public int id { get; set; }
        public string name { get; set; }
        public int depart_id { get; set; }
        [NotMapped]
        public List<Student> students { get; set; }

    }

[Table("subject")]
    public class Subject
    {
        public int id { get; set; }
        public string name { get; set; }
        public int depart_id { get; set; }
        [NotMapped]
        public List<Teacher> teachers { get; set; }
        [NotMapped]
        public List<Student> students { get; set; }
    }

[Table("depart")]
    public class Department
    {
        public int id { get; set; }
        public string name { get; set; }

        [NotMapped]
        public List<Address> addresses { get; set; }
        [NotMapped]
        public List<Grade> grades { get; set; }
        [NotMapped]
        public List<Subject> subjects { get; set; }
        [NotMapped]
        public List<Teacher> teachers { get; set; }
    }

[Table("address")]
    public class Address
    {
        public int id { get; set; }
        public string country { get; set; }
        public string city { get; set; }
        [NotMapped]
        public List<Department> departs { get; set; }
        
    }

类之间的关系如表1描述。

表1

Department与Grade, Subject, Teacher之间都是一对多的关系。以Teacher为例，Department与Teacher之间的对应关系是通过depart_id外键实现的，那么在构建Department模型时，该关系用代码描述为：

builder.Entity<Department>()
                .HasMany(d => d.teachers)
                .WithOne(t => t.depart).HasForeignKey(t => t.depart_id);

其中t=>t.depart可以省略，即

builder.Entity<Department>()
                .HasMany(d => d.teachers)
                .WithOne().HasForeignKey(t => t.depart_id);

Department与Address之间是多对多的关系，这种关系映射在数据库时通常需要有一张关系表，用于记录Department和Address之间的关系。同样地，要给这张关系表也建立一个相应的类：

[Table("depart_address")]
    public class Depart_Address
    {
        public int id { get; set; }
        public int depart_id { get; set; }
        public int address_id { get; set; }

        [NotMapped]
        public Department depart { get; set; }
        [NotMapped]
        public Address address { get; set; }
    }
        

多对多的关系模型描述如下：

builder.Entity<Department>()
                .HasMany(d => d.addresses)
                .WithMany(a => a.departs)
                .UsingEntity<Depart_Address>(
                j => j.HasOne(pt => pt.address)
                .WithMany().HasForeignKey(pt => pt.address_id),
                j => j.HasOne(pt => pt.depart)
                .WithMany().HasForeignKey(pt => pt.depart_id),
                j => j.HasKey(t => new { t.depart_id, t.address_id }));

整段模型代码为：

protected override void OnModelCreating(ModelBuilder builder)
        {
            #region Student Relationships
            // students-teachers : multi-to-multi
            builder.Entity<Student>()
                .HasMany(s => s.teachers)
                .WithMany(t => t.students)
                .UsingEntity<StudentTeacher>(
                j => j.HasOne(pt => pt.teacher)
                .WithMany()
                .HasForeignKey(pt => pt.teacher_id),
                j => j.HasOne(pt => pt.student)
                .WithMany()
                .HasForeignKey(pt => pt.student_id),
                j => j.HasKey(t => new { t.student_id, t.teacher_id }));

            // students-subjects: multi-to-multi
            builder.Entity<Student>()
                .HasMany(s => s.subjects)
                .WithMany(su => su.students)
                .UsingEntity<StudentSubject>(
                j => j.HasOne(pt => pt.subject)
                .WithMany()
                .HasForeignKey(pt => pt.subject_id),
                j => j.HasOne(pt => pt.student)
                .WithMany()
                .HasForeignKey(pt => pt.student_id),
                j => j.HasKey(t => new { t.student_id, t.subject_id }));

            // student-address: multi-to-one
            builder.Entity<Student>()
                .HasOne(s => s.address)
                .WithMany()
                .HasForeignKey(s => s.address_id);
            #endregion

            #region Teacher Relationships
            // teachers-address: multi-to-one
            builder.Entity<Teacher>()
                .HasOne(t => t.address)
                .WithMany()
                .HasForeignKey(t => t.address_id);

            builder.Entity<Teacher>()
                .HasMany(t => t.students)
                .WithMany(s => s.teachers)
                .UsingEntity<StudentTeacher>(
                j => j.HasOne(pt => pt.student)
                .WithMany().HasForeignKey(pt => pt.student_id),
                j => j.HasOne(pt => pt.teacher)
                .WithMany().HasForeignKey(pt => pt.teacher_id),
                j => j.HasKey(t => new { t.teacher_id, t.student_id }));
            #endregion

            #region Department Relationships
            // department-grades: one-to-multi
            builder.Entity<Department>()
                .HasMany(d => d.grades)
                .WithOne().HasForeignKey(g => g.depart_id);

            // department-teachers: one-to-multi
            builder.Entity<Department>()
                .HasMany(d => d.teachers)
                .WithOne().HasForeignKey(t => t.depart_id);

            // department-subjects: one-to-multi
            builder.Entity<Department>()
                .HasMany(d => d.subjects)
                .WithOne().HasForeignKey(s => s.depart_id);

            // departments-addresses: multi-to-multi
            builder.Entity<Department>()
                .HasMany(d => d.addresses)
                .WithMany(a => a.departs)
                .UsingEntity<Depart_Address>(
                j => j.HasOne(pt => pt.address)
                .WithMany().HasForeignKey(pt => pt.address_id),
                j => j.HasOne(pt => pt.depart)
                .WithMany().HasForeignKey(pt => pt.depart_id),
                j => j.HasKey(t => new { t.depart_id, t.address_id }));
            #endregion

            #region Grade Relationships
            // grade-students: one-to-multi
            builder.Entity<Grade>()
                .HasMany(g => g.students)
                .WithOne().HasForeignKey(s => s.grade_id);
            #endregion

            #region Subject Relationships
            // subjects-students: multi-to-multi
            builder.Entity<Subject>()
                .HasMany(s => s.students)
                .WithMany(stu => stu.subjects)
                .UsingEntity<StudentSubject>(
                j => j.HasOne(pt => pt.student)
                .WithMany()
                .HasForeignKey(pt => pt.student_id),
                j => j.HasOne(pt => pt.subject)
                .WithMany()
                .HasForeignKey(pt => pt.subject_id),
                j => j.HasKey(t => new { t.subject_id, t.student_id }));

            builder.Entity<Subject>()
                .HasMany(s => s.teachers)
                .WithOne().HasForeignKey(t => t.subject_id);
            #endregion
        }

测试：

1.构建Department对象

Department depart = context.departs
                .Include(d => d.subjects)
                .ThenInclude(s=>s.students)
                .ThenInclude(s=>s.teachers)
                .Include(d=>d.addresses)
                .Include(d => d.teachers)
                .ThenInclude(t=>t.students)
                .Include(d => d.grades)
                .ThenInclude(g => g.students).First();

Console.WriteLine("department name: " + depart.name);
            Console.WriteLine();
            Console.WriteLine("department subjects:");
            foreach(var item in depart.subjects)
            {
                Console.WriteLine((depart.subjects.IndexOf(item)+1).ToString()+"." + item.name);
                Console.WriteLine("Students choose this subject:");
                foreach (var stu in item.students)
                    Console.Write(stu.name+",");
                Console.WriteLine();
                Console.WriteLine("Teachers who teach this subject:");
                foreach (var t in item.teachers)
                    Console.Write(t.name+",");
                Console.WriteLine();
            }
            Console.WriteLine();
            Console.WriteLine("department teachers:");
            foreach(var item in depart.teachers)
            {
                Console.WriteLine((depart.teachers.IndexOf(item)+1).ToString()+"."+item.name);
                Console.WriteLine("his or her students:");
                foreach (var stu in item.students)
                    Console.Write(stu.name + ",");
                Console.WriteLine();
            }
            Console.WriteLine();
            Console.WriteLine("department grades:");
            foreach(var item in depart.grades)
            {
                Console.WriteLine((depart.grades.IndexOf(item) + 1).ToString() + "." + item.name);
                Console.WriteLine("students in this grade:");
                foreach (var stu in item.students)
                    Console.Write(stu.name + ",");
                Console.WriteLine();
            }
            Console.WriteLine();
            Console.WriteLine("department addresses:");
            foreach(var item in depart.addresses)
            {
                Console.WriteLine(item.country + "." + item.city);
            }

测试结果：

2.构建Students对象

List<Student> students = context.students
                .Include(s => s.subjects)
                .Include(s => s.teachers)
                .Include(s => s.address).ToList();

Console.WriteLine("Students List:");
            foreach(var item in students)
            {
                Console.WriteLine((students.IndexOf(item) + 1).ToString() + "." + item.name);
                Console.WriteLine("his or her subjects:");
                foreach(var sub in item.subjects)
                {
                    Console.Write(sub.name + ",");
                }
                Console.WriteLine();
                Console.WriteLine("his or her teachers:");
                foreach (var t in item.teachers)
                    Console.Write(t.name + ",");
                Console.WriteLine();
                Console.WriteLine("his or her address:" + item.address.country + "." + item.address.city);
            }

 完整代码路径：

https://github.com/Larissa1990/EFcore_demo