Associate ApplicationUser with Model

Question

I'm developing a simple MVC 5 application using Entity Framework 6 in Visual Studio 2013. I want to associate users (from ASP.NET Identity 2.0) with my own models in the following manner:

There are two types of users (roles), an author and a grader. The author posts an article and the grader generates a grade report for such article. Any grader can grade any ungraded article. The author will be able to see who graded their articles, so I'm trying to associate 2 users with an article (as well as with grade reports).

I want to know if I'm on the right direction or if I should add or modify something. I've read something about overriding the onModelCreating method so I don't get some errors related with keys, but I don't understand that very well.

These are my models. As you can see, there's a one-to-zero-or-one relationship between an Article and a GradeReport. A GradeReport only exists in relation to one Article.

public class Article
{
    public int ArticleID { get; set; }
    public string Title { get; set;  }
    public string Content { get; set; }
    public DateTime Date { get; set; }
    public bool Status { get; set; } // To tell if it has been graded or not
    public string AuthorID { get; set; }
    public string? GraderID { get; set; }
    public int? GradeReportID { get; set; }

    public virtual ApplicationUser Author { get; set; }
    public virtual ApplicationUser Grader { get; set; }
    public virtual GradeReport GradeReport { get; set; }
}

public class GradeReport 
{
    [Key, ForeignKey("Article")]
    public int ArticleID { get; set;}
    public DateTime Date { get; set; }
    public string Grade { get; set; }
    public string Comments { get; set; }
    public string GraderID { get; set; }
    public string AuthorID { get; set; }

    public virtual Article Article { get; set; }
    public virtual ApplicationUser Grader { get; set; }
    public virtual ApplicationUser Author { get; set; }
}

This is my ApplicationUser class. I added some custom properties which include the articles and grade reports for a particular user.

public class ApplicationUser : IdentityUser
{
    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
    {
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        return userIdentity;
    }

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<Article> Articles { get; set; }
    public virtual ICollection<GradeReport> GradeReports { get; set; }
}

This is my DB context which extends IdentityDbContext:

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public ApplicationDbContext()
        : base("DefaultConnection", throwIfV1Schema: false)
    {
    }

    static ApplicationDbContext()
    {
        Database.SetInitializer<ApplicationDbContext>(new ApplicationDbInitializer());
    }

    public static ApplicationDbContext Create()
    {
        return new ApplicationDbContext();
    }

    DbSet<Article> Articles { get; set; }
    DbSet<GradeReport> GradeReports { get; set; }
}

Am I doing it right? Is there a better way to associate users with models?