EF 5.0 Trouble updating entity which is already tracked - asp.net

Question

I'll preface this question with the following: I know there are a million posts on the internet about the old "An object with the same key already exists in the ObjectStateManager" issue. My scenario is a bit more complicated, I think.
I have a UnitOfWork class which creates a DbContext and passes it to any repository which is called. The pattern I'm using closely follows the Unit of Work tutorial on the ASP.NET site. Unlike the tutorial, my repositories take in Business entities, map them to data entities, and perform some CRUD action. My Business logic only works with Business entities. Here is what I'm trying to do in a sample Business Manager class:
_unitOfWork.Repository.Add(entity);
_unitOfWork.Save(); // context.SaveChanges() under the hood
...Perform some operations on the model...
_unitOfWork.Repository.Update(entity);
_unitOfWork.Save();
Here is a sample Update method from the repository:
public virtual void Update(entity)
{
var dataEntity = // map from business entity to data;
_context.Entry(dataEntity).State = EntityState.Modified;
}
It obviously fails on the last line. Here is where my confusion sets in:
The entity's State is Detached
When I attempt to change the State to Modified or Unchanged, it gives me the ObjectStateManager exception above.
When I attempt to detach the entity from the context (((IObjectContextAdapter)_context).ObjectContext.Detach(entity);) I get an exception about how the entity is not attached to the context, therefore, it cannot detach it. Very confusing (something fundamental I'm missing, for sure).
Many other posts suggest I make a database call, update that entity in the repository, then _unitOfWork.Save(). I don't like this approach. I shouldn't need to make an unnecessary network call to update an entity.
The Update method in the repository needs to handle two scenarios: 1) updating an entity which is not currently tracked by the context, and 2) updating an entity which IS currently tracked by the context. The second piece is what I'm struggling with.
Any help or insight is appreciated.
Thanks!

Answer 1

This means that there already is an object attached to the context with the same key as the new dataEntity. The existing object and the new entity both represent the same entry in the database but they are two different objects.
This may indicate that the lifespan of your _context is too long, but that's hard to judge from your code. It is certain though that the context was previously used to fetch an entity from the database that is subsequently duplicated by var dataEntity = ....
You may have to shorten the lifespan of the context, I can't tell. If you think it's OK you may want to use the Local collection to check whether the entity is already there. That will save the database round trip that Find may still make.

Answer 2

I found a hybrid solution which appears to work:
public virtual void Update(TB entity)
{
var dataEntity = Mapper.Map<TB, TD>(entity);
var pkey = _dbSet.Create().GetType().GetProperty("Id").GetValue(dataEntity);
var entry = _context.Entry(dataEntity);
if (entry.State == EntityState.Detached)
{
var attachedEntity = _dbSet.Find(pkey);
if (attachedEntity != null)
{
var attachedEntry = _context.Entry(attachedEntity);
attachedEntry.CurrentValues.SetValues(dataEntity);
}
else
{
entry.State = EntityState.Modified;
}
}
else
{
entry.State = EntityState.Modified;
}
}