Entity Framework常用方法及案例

⒈Skip(int count)

　　说明：跳过集合的前n个元素；延迟。即我们跳过给定的数目返回后面的结果集。

⒉Take(int count)

　　说明：获取集合的前n个元素；延迟。即只返回限定数量的结果集。

案例：分页查询

        public IList<StudentScore> test1(int pageSize,int pageIndex)
        {
            //skip表示跳过多少条，Take表示取多少条
            var list = _db.StudentScore.Skip(pageSize * (pageIndex - 1)).Take(pageSize).ToList();
            return list;
        }

⒊OrderBy

说明：按指定表达式对集合排序；延迟，默认是升序，加上descending表示降序，对应的扩展方法是 OrderBy和OrderByDescending

        public IList<StudentScore> test2()
        {
            var list = _db.StudentScore.OrderBy(s => s.Id).ToList();
            var list2 = _db.StudentScore.OrderByDescending(s => s.Id).ToList();
            return list;
        }

⒋Add

说明：添加一个实体

        public int test4()
        {
            var student = new Student {SName = "fanqi",SSex = 1,SBirthday = new DateTime(1994,6,24) };
            var add = _db.Student.Add(student);
            _db.SaveChanges();
            return add.Entity.Id;
        }

⒌Remove

说明：删除一个实体

        public void test5()
        {
            _db.Remove(new Student { Id = 9 });
            _db.SaveChanges();

            //Student user = new Student(){ Id = 9};
            //将对象添加到EF管理容器
            //_db.Student.Attach(user);
            //将对象包装类的标识状态变更为删除状态
            //_db.Student.Remove(user);
            //通知上下文将实体的变化保存到数据库中
            //_db.SaveChanges();
        }

 ⒍Update

说明：更新一个实体

        public void test6()
        {
            Student student = _db.Student.FirstOrDefault(f => f.Id == 7);
            student.SName = "fanqi";
            _db.Student.Update(student);
            _db.SaveChanges();

            //声明一个实体对象并赋初始值
            //Student student = new Student { Id = 8, SName = "fanqi" };
            //将实体对象加入到EF对象容器中，并获取伪包装类对象
            //EntityEntry<Student> entry = _db.Entry<Student>(student);
            //将包装类对象的状态设置为unchanged
            //entry.State = EntityState.Unchanged;
            //设置实体对象被改变的属性
            //entry.Property(a => a.SName).IsModified = true;
            //通知上下文将实体的变化保存到数据库中
            //_db.SaveChanges();
        }

⒎Join

说明：内连接

        public dynamic test7()
        {
            var studentList = _db.Student;
            var scoreList = _db.StudentScore;
            var newList = studentList.Join(scoreList, stu => stu.Id, sco => sco.SId, (stu, sco) => new
            {
                studentId = stu.Id,
                studentName = stu.SName,
                studentSex = stu.SSex == 1?"男":"女",
                scoreId = sco.Id,
                courseId = sco.CId,
                courseScoreNum = sco.Score
            });
            return newList;
        }

⒏GroupJoin

说明：左连接

        public dynamic test8()
        {
            var studentList = _db.Student;
            var scoreList = _db.StudentScore;

            var newList = studentList.GroupJoin(scoreList, stu => stu.Id, sco => sco.SId, (stu, sco) => new
            {
                studentId = stu.Id,
                studentName = stu.SName,
                studentSex = stu.SSex == 1 ? "男" : "女",
                sco
            });
            return newList;
        }