Multiplicity error when generating database from model?

have 3 classes:

     public class User
    {
        public 开发者_C百科Guid Id { get; set; }
        public string Username { get; set; }
        public string PersonelNumber { get; set; }
        public string Password { get; set; }
        public bool IsActive { get; set; }
        public DateTime LastLoginDate { get; set; }
        public bool IsOnline { get; set; }
    }

    public class Comment
    {
        public Guid Id { get; set; }

        /// <summary>
        /// Title of the comment
        /// </summary>
        public string Title { get; set; }

        /// <summary>
        /// Full text for comment
        /// </summary>
        public string Text { get; set; }

        //Maps to the User Id
        public Guid CreatedById { get; set; }

        public virtual User CreatedBy { get; set; }
    }

    public class FileVersion 
    {
        public Guid Id { get; set; }
        public string Path { get; set; }
        public int Version { get; set; }

//Referecne to optional comment        
        public virtual Guid ChangeCommentId { get; set; }
        public virtual Comment ChangeComment { get; set; }
//Referece to user who added a file
        public virtual Guid AddedById { get; set; }
        public virtual User AdddedBy { get; set; }
//Reference to User who may have the file checked out
        public virtual Guid CheckedOutById { get; set; }
        public virtual User CheckedOutBy { get; set; }
}

I am receiving the error when the database tries to create the Comment relationship between FilVersion and Comment. I did create this mapping for the FileVersion class:

HasOptional(f => f.ChangeComment).WithMany().HasForeignKey(f => f.ChangeCommentId).WillCascadeOnDelete(false);

I am not sure how to setup my mappings so that I do not get the Multiplicity error when the database is created. The FileVersion needs a referecne to who added the file and who may have it checkedout for edit.

---

The problem is that you cannot define Comment as optional when the foreign key doesn't accept null. Either modify your entity to:

public class FileVersion 
{
    ...     
    public virtual Guid? ChangeCommentId { get; set; }
    public virtual Comment ChangeComment { get; set; }
    ...      
}

Or modify your mapping to:

modelBuilder.Entity<FileVersion>()
            .HasRequired(f => f.ChangeComment)
            .WithMany()
            .HasForeignKey(f => f.ChangeCommentId)
            .WillCascadeOnDelete(false);

Same will be probably needed for CheckedOutBy