System.Data.Entity.Edm.EdmEnti...

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MVC4

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;
using System.Linq;
using System.Web;

namespace MVCEF.Models
{

    public class StudentContext : DbContext
    {
        public DbSet<StudentModel> Students { get; set; }

    }
    public class StudentModel
    {
        [Key]
        public int Roll_No;
        //public int Roll_No { get; set; }
        public string Name { get; set; }
        public string Mobile_No { get; set; }
        public string City { get; set; }
    }
}

this is my model class.when i m trying to add a controller in
model class :StudentModel (MVCEF.Models)
and datacontext file :StudentContext (MVCEF.Models)

it give me the following error

System.Data.Entity.Edm.EdmEntityType has no key defined

plz help me