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 - )).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 = ,SBirthday = new DateTime(,,) };
             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 =  });
             _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 == );
             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 == ?"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 ==  ? "男" : "女",
                 sco
             });
             return newList;
         }