How to update complex model in ASP.NET MVC 3

I am trying to update a complex model in a single view. I am using ASP.NET MVC3, Entity Framework with Code first, unit of work, generic repository pattern.. but when I try to update the model, i come up with this error:

A referential integrity constraint violation occurred: The property values that define the referential constraints are not consistent between principal and dependent objects in the relationship.

Here is my simplified view model:

public class TransactionViewModel
{
     public Transaction Transaction { get; set; }
     public bool IsUserSubmitting { get; set; }
     public IEnumerable<SelectListItem> ContractTypes { get; set; }
}

Here is my simplified complex model, and as an example one of its navigation property. Transaction has one to one relationship with all of its navigation properties:

public class Transaction
{
    [Key]
    public int Id { get; set; }

    public int CurrentStageId { get; set; }

    public int? BidId { get; set; }

    public int? EvaluationId { get; set; }

    public virtual Stage CurrentStage { get; set; }

    public virtual Bid Bid { get; set; }

    public virtual Evaluation Evaluation { get; set; }

}

public class Bid
{
    [Key]
    public int Id { get; set; }

    public string Type { get; set; }

    public DateTime? PublicationDate { get; set; }

    public DateTime? BidOpeningDate { get; set; }

    public DateTime? ServiceDate { get; set; }

    public string ContractBuyerComments { get; set; }

    public string BidNumber { get; set; }

    public DateTime? ReminderDate { get; set; }

    public DateTime? SubmitDate { get; set; }

}

Using the same view model, I am able to create a transaction object, which would populate the database like this.

Id: 1, CurrentStageId: 1, BidId: 1, EvaluationId: 1

but, when I try to update properties within these navigation properties, this line causes the error, in controller:

[HttpPost]
public ActionResult Edit(TransactionViewModel model)
{
    if (ModelState.IsValid)
    {
        -> unitOfWork.TransactionRepository.Update(model.Transaction);
           unitOfWork.Save();
           return RedirectToAction("List");
    }
}

In generic repository:

public virtual void Update(TEntity entityToUpdate)
{
 -> dbSet.Attach(entityToUpdate);
    context.Entry(entityToUpdate).State = EntityState.Modified;
}

The problem is further complicated because I should be able to edit any of the fields(properties) within any of the navigation property within Transaction object within a single view.