ASP.NET MVC: EF 没有创建数据库表

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.Entity;
using MVC_example.Models;

namespace MVC_example.DataAccessLayer
{
    public class Cnn : DbContext
    {
        private DbSet<User> _users;

        public DbSet<User> Users
        {
            get
            {
                return _users;
            }

            set
            {
                _users = value;
            }
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<User>().ToTable("tUser");

            base.OnModelCreating(modelBuilder);
        }
    }
}

<connectionStrings>
    <add connectionString="Data Source=.;Initial Catalog=SalesERPDB;Integrated Security=True;" name="Cnn" providerName="System.Data.SqlClient" />
  </connectionStrings>