EF CodeFirst之四大初始化策略

随着业务的发展，我们在开发当中可能会添加字段，添加表。怎么来解决了？CodeFirst给我们提供了一下四大策略：
官方文档：http://www.entityframeworktutorial.net/code-first/database-initialization-strategy-in-code-first.aspx

1.CreateDatabaseIfnotExists (默认的策略)

特性：1.数据库不存在，创建数据库

         2.model修改，执行会抛出异常

这是开发当中最稳定的一种方式，不会存在说数据库里面的数据突然被删除。

    public class SchoolDB2Entities : DbContext
    {
        public SchoolDB2Entities() : base("data source=.;initial catalog=SchoolDB2;integrated security=True")
        {
       //数据库不存在，创建数据库，model修改，执行会抛出异常
           Database.SetInitializer<SchoolDBContext>(new CreateDatabaseIfNotExists<SchoolDBContext>());
        }

        public DbSet<Student> Students { get; set; }

        public DbSet<StudentAddress> StudentAddresses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {

        }
    }

2.DropCreateDatabaseIfModelChanges

特性：一旦model修改了，将会执行dropdatabase(删除数据库)操作。

缺点：一旦你有原始数据，那么对不起，都会被删除。（所以在生成环境中，谨慎使用）

 public class SchoolDB2Entities : DbContext
    {
        public SchoolDB2Entities() : base("data source=.;initial catalog=SchoolDB2;integrated security=True")
        {
            //model一旦修改，db进行rebuild
            Database.SetInitializer<SchoolDB2Entities>(new DropCreateDatabaseIfModelChanges<SchoolDB2Entities>());
        }

        public DbSet<Student> Students { get; set; }

        public DbSet<StudentAddress> StudentAddresses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {

        }
    }

3.DropCreateDatabaseAlways

特性：一直都是rebuild(重建)操作，db每次都会重建。

 public class SchoolDB2Entities : DbContext
    {
        public SchoolDB2Entities() : base("data source=.;initial catalog=SchoolDB2;integrated security=True")
        {
            //一直都是rebuild操作。。。
             Database.SetInitializer<SchoolDB2Entities>(new DropCreateDatabaseAlways<SchoolDB2Entities>());
        }

        public DbSet<Student> Students { get; set; }

        public DbSet<StudentAddress> StudentAddresses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {

        }
    }

4.Custom DB Initializer

特性：自定义初始化，重写override,可以做一些种子数据。。(数据库的基础数据)

namespace ConsoleApplication19
{
    public class SchoolDB2Entities : DbContext
    {
        public SchoolDB2Entities() : base("data source=.;initial catalog=SchoolDB2;integrated security=True")
        {
            //用自定义的数据库初始化类
            Database.SetInitializer<SchoolDB2Entities>(new MyCustomIntializer());
        }

        public DbSet<Student> Students { get; set; }

        public DbSet<StudentAddress> StudentAddresses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {

        }
        //自定义的数据库初始化类
        public class MyCustomIntializer : DropCreateDatabaseAlways<SchoolDB2Entities>
        {
            public override void InitializeDatabase(SchoolDB2Entities context)
            {
                base.InitializeDatabase(context);
            }

            //重写seed，追加初始数据
            protected override void Seed(SchoolDB2Entities context)
            {
                for (int i = 0; i < 10; i++)
                {
                    context.Students.Add(new Student()
                    {
                        StudentName = "mary" + i
                    });
                }

                context.SaveChanges();

                base.Seed(context);
            }
        }
    }
}

那么最后，当在正式环境当中，怎么禁用这些初始化策略呢？

 public class SchoolDB2Entities : DbContext
    {
        public SchoolDB2Entities() : base("data source=.;initial catalog=SchoolDB2;integrated security=True")
        {
            //禁用数据库初始化策略
             Database.SetInitializer<SchoolDB2Entities>(new NullDatabaseInitializer<TContext>());
        }

        public DbSet<Student> Students { get; set; }

        public DbSet<StudentAddress> StudentAddresses { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {

        }
    }

其实用EF的目的，就是写更少的代码，你甚至可以不用了解db。现在的问题是当model改变，怎么办？这就用到了CodeFirst数据库迁移。