ApplicationDbContext does not exist in current context?

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Microsoft.Owin;
using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.Owin;
using Microsoft.Owin.Security.Cookies;
using Microsoft.Owin.Security.Google;
using Owin;
using eNtsaTrainingRegistration.Models;

namespace eNtsaTrainingRegistration.App_Start
{
    public partial class Startup
    {

        public void ConfigureAuth(IAppBuilder app)
        {
            // configure the db context, user manager and signin manager to a single instance per request.

            app.CreatePerOwinContext(ApplicationDbContext);
            app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
            app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);
            app.CreatePerOwinContext<ApplicationRoleManager>(ApplicationRoleManager.Create);



            // Configure sign cookie.
            app.UseCookieAuthentication(new CookieAuthenticationOptions
            {
                AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
                LoginPath = new PathString("/login"),
                Provider = new CookieAuthenticationProvider
                {
                    // Enables the application to validate the security stamp when the user logs in.
                    // This is a security feature which is used when you change a password or add an external login to your account.
                    OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, ApplicationUser>(
                      validateInterval: TimeSpan.FromMinutes(30),
                      regenerateIdentity: (manager, user) => user.GenerateUserIdentityAsync(manager))
                }
            });
            app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);

            // Enables app to temperorily store user information when they are verified.
            app.UseTwoFactorSignInCookie(DefaultAuthenticationTypes.TwoFactorCookie, TimeSpan.FromMinutes(5));

            // To remember when to login in.
            app.UseTwoFactorRememberBrowserCookie(DefaultAuthenticationTypes.TwoFactorRememberBrowserCookie);
        }

    }
}