在Entity Framework中,Seed在EF 4.1中引入,并与数据库初始化程序一起使用. 种子方法的一般概念是将数据初始化为由Code First创建或由迁移演变的数据库.这些数据通常是测试数据,但也可能是参考数据,例如已知学生,课程等的列表.当数据初始化时,它会执行以下和减去;
检查目标数据库是否已存在.
如果确实如此,则将当前Code First模型与存储在元数据中的模型进行比较.数据库.
如果当前模型与数据库中的模型不匹配,则删除数据库.
如果数据库被删除或创建数据库,则会创建数据库.首先不存在.
如果创建了数据库,则调用初始化程序Seed方法.
Seed方法将数据库上下文对象作为输入参数,方法中的代码使用该对象将新实体添加到数据库中.要将数据植入数据库,您需要覆盖Seed方法.让我们看一下下面的示例,其中一些默认数据在内部类中启动到数据库中.
private class UniDBInitializer<T> : DropCreateDatabaseAlways<MyContext> { protected override void Seed(MyContext context) { IList<Student> students = new List<Student>(); students.Add(new Student() { FirstMidName = "Andrew", LastName = "Peters", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); students.Add(new Student() { FirstMidName = "Brice", LastName = "Lambson", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); students.Add(new Student() { FirstMidName = "Rowan", LastName = "Miller", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); foreach (Student student in students) context.Students.Add(student); base.Seed(context); } }
在上面的代码中,学生表被初始化.您需要在上下文类中设置此DB初始化程序类,如以下代码所示.
public MyContext() : base("name=MyContextDB") { Database.SetInitializer<MyContext>(new UniDBInitializer<MyContext>()); }
以下是MyContext类的完整类实现,它还包含DB初始化类.
public class MyContext : DbContext { public MyContext() : base("name=MyContextDB") { Database.SetInitializer<MyContext>(new UniDBInitializer<MyContext>()); } public virtual DbSet<Course> Courses { get; set; } public virtual DbSet<Enrollment> Enrollments { get; set; } public virtual DbSet<Student> Students { get; set; } private class UniDBInitializer<T> : DropCreateDatabaseAlways<MyContext> { protected override void Seed(MyContext context) { IList<Student> students = new List<Student>(); students.Add(new Student() { FirstMidName = "Andrew", LastName = "Peters", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); students.Add(new Student() { FirstMidName = "Brice", LastName = "Lambson", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); students.Add(new Student() { FirstMidName = "Rowan", LastName = "Miller", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }); foreach (Student student in students) context.Students.Add(student); base.Seed(context); } } }
编译并执行上述示例时,可以看到数据库中的数据如下图所示.
我们建议为了更好地理解,您可以逐步执行上述示例.