ORM开发（2）-EF必杀神器

我们先从Code First模式出发，我们先创建三个类，来代表学生和课程的关系——Student，Course，Enrollment。有人可能会问，Enrollment是干嘛的？这是我们为了解耦所做的工作，如果我们单纯的让学生拥有多个课程，而一个课程有多个学生报名，那么就会形成多对多的关系，如果我们在其中添加一个中间层，那么则会形成多个一对一关系，从而完成解耦。

public class Student
    {
        public int ID { get; set; }
        public string LastName { get; set; }
        public string FirstMidName { get; set; }
        public DateTime EnrollmentDate { get; set; }

        public virtual ICollection<Enrollment> Enrollments { get; set; }
    }

    public enum Grade
    {
        A, B, C, D, F
    }

    public class Enrollment
    {
        public int EnrollmentID { get; set; }
        public int CourseID { get; set; }
        public int StudentID { get; set; }
        public Grade? Grade { get; set; }

        public virtual Course Course { get; set; }
        public virtual Student Student { get; set; }
    }

    public class Course
    {
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int CourseID { get; set; }
        public string Title { get; set; }
        public int Credits { get; set; }

        public virtual ICollection<Enrollment> Enrollments { get; set; }
    }

之后，我们创建一个继承自DbContext的类，这个类主要承担起和数据库的交互：

public class SchoolContext:DbContext
    {
        public SchoolContext() : base("SchoolContext")
        {
        }

        public DbSet<Student> Students { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }
        public DbSet<Course> Courses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        }
    }

OnModelCreating事件是在创建数据库中表时触发的，在方法中，我们只做了一件事，那就是让表的名字从复数变成单数。

虽然，我们启动时会在数据库中创建三个表，但是我们还没有数据，而且我们也没说连接哪个数据库。好吧，我们先写下连接字符串，前面已经说过了，这里贴一下就好了，EF自动调用它的：

<connectionStrings>
    <add name="SchoolContext" connectionString="data source=BLACKERXHUNTER\SQLEXPRESS;Initial Catalog=ContosoUniversity;Integrated Security=True;" providerName="System.Data.SqlClient" />
  </connectionStrings>

然后，我们再创建数据：

public class SchoolInitializer : System.Data.Entity.DropCreateDatabaseIfModelChanges<SchoolContext>
    {
        protected override void Seed(SchoolContext context)
        {
            var students = new List<Student>
            {
            new Student{FirstMidName="Carson",LastName="Alexander",EnrollmentDate=DateTime.Parse("2005-09-01")},
            new Student{FirstMidName="Meredith",LastName="Alonso",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Arturo",LastName="Anand",EnrollmentDate=DateTime.Parse("2003-09-01")},
            new Student{FirstMidName="Gytis",LastName="Barzdukas",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Yan",LastName="Li",EnrollmentDate=DateTime.Parse("2002-09-01")},
            new Student{FirstMidName="Peggy",LastName="Justice",EnrollmentDate=DateTime.Parse("2001-09-01")},
            new Student{FirstMidName="Laura",LastName="Norman",EnrollmentDate=DateTime.Parse("2003-09-01")},
            new Student{FirstMidName="Nino",LastName="Olivetto",EnrollmentDate=DateTime.Parse("2005-09-01")}
            };

            students.ForEach(s => context.Students.Add(s));
            context.SaveChanges();
            var courses = new List<Course>
            {
            new Course{CourseID=1050,Title="Chemistry",Credits=3,},
            new Course{CourseID=4022,Title="Microeconomics",Credits=3,},
            new Course{CourseID=4041,Title="Macroeconomics",Credits=3,},
            new Course{CourseID=1045,Title="Calculus",Credits=4,},
            new Course{CourseID=3141,Title="Trigonometry",Credits=4,},
            new Course{CourseID=2021,Title="Composition",Credits=3,},
            new Course{CourseID=2042,Title="Literature",Credits=4,}
            };
            courses.ForEach(s => context.Courses.Add(s));
            context.SaveChanges();
            var enrollments = new List<Enrollment>
            {
            new Enrollment{StudentID=1,CourseID=1050,Grade=Grade.A},
            new Enrollment{StudentID=1,CourseID=4022,Grade=Grade.C},
            new Enrollment{StudentID=1,CourseID=4041,Grade=Grade.B},
            new Enrollment{StudentID=2,CourseID=1045,Grade=Grade.B},
            new Enrollment{StudentID=2,CourseID=3141,Grade=Grade.F},
            new Enrollment{StudentID=2,CourseID=2021,Grade=Grade.F},
            new Enrollment{StudentID=3,CourseID=1050},
            new Enrollment{StudentID=4,CourseID=1050,},
            new Enrollment{StudentID=4,CourseID=4022,Grade=Grade.F},
            new Enrollment{StudentID=5,CourseID=4041,Grade=Grade.C},
            new Enrollment{StudentID=6,CourseID=1045},
            new Enrollment{StudentID=7,CourseID=3141,Grade=Grade.A},
            };
            enrollments.ForEach(s => context.Enrollments.Add(s));
            context.SaveChanges();
        }
    }

好吧，这个类稍稍有点长，不过其实结构挺简单的，我们创建了一个继承自DropCreateDatabaseIfModelChanges的泛型类，请把这个类的名字都出来——如果已经存在了一个同名的表在就重建一个表。嗯，就是这个意思。之后我们重载了seed方法，然后创建数据。对于每一个数据，我们调用了SchoolContext类里面的Set集合的Add方法，并保存，你可能会问我们为什么要些三遍SaveChanges，那是因为如果有哪个报错，就知道是哪里错了，它的使用也是超级简单的，只要在需要调用数据的地方添加这行代码即可：

private SchoolContext db = new SchoolContext();

之后随意改数据，删数据都不在话下~！