命名空间中不存在类型或命名空间名称

我在.cs文件中有下面的代码，我遇到了下面的错误，任何帮助将不胜感激。

using System.Data.Entity;
namespace Assignment2.Models
{
    public class TicketBookingEntities : DbContext
    {
        public DbSet<Performance> Performances { get; set; }
        public DbSet<Production> Productions { get; set; }
    }
}

错误：

Error   1   The type or namespace name 'Entity' does not exist in the namespace 'System.Data' (are you missing an assembly reference?)
Error   2   The type or namespace name 'DbContext' could not be found (are you missing a using directive or an assembly reference?)
Error   3   The type or namespace name 'DbSet' could not be found (are you missing a using directive or an assembly reference?)
Error   4   The type or namespace name 'DbSet' could not be found (are you missing a using directive or an assembly reference?)