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What is the main reason for the exception of this RelationshipExpate exception in the ReliantEndEnd exception in the Entity Framework?

I am using Entity Framework 4.3.1 from an ASP.NET MVC 3.0 web application, and I have some code that separates the old object and replaces it with a new object that looks like this:

public void UpdateUnattached(T entryToUpdate, T updatedEntry)
{
    var ctx = (EntityFrameworkContext) _context;           
    ctx.ChangeObjectState(EntityState.Detached, entryToUpdate);
    _set.Attach(updatedEntry);       
    ctx.ChangeObjectState(EntityState.Modified, updatedEntry);
}

This works all the time, except when I get an exception below in the line _set.Attach.

You cannot return this RelationshipManager. The RelatedEnd function can only be returned by the MaterialManager for objects that are tracked by the ObjectStateManager or implement IEntityWithRelationships.

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(InnerException is null)...

Type: System.InvalidOperationException 
Source: System.Data.Entity 
Target site: GetRelatedEndInternal 
Stacktrace: at System.Data.Objects.DataClasses.RelationshipManager.GetRelatedEndInternal(String relationshipName, String targetRoleName) 
at System.Data.Objects.EntityEntry.FindRelatedEntityKeysByForeignKeys(Dictionary`2& relatedEntities, Boolean useOriginalValues) 
at System.Data.Objects.EntityEntry.TakeSnapshotOfForeignKeys() 
at System.Data.Objects.ObjectStateManager.PromoteKeyEntryInitialization(EntityEntry keyEntry, IEntityWrapper wrappedEntity, IExtendedDataRecord shadowValues, Boolean replacingEntry) 
at System.Data.Objects.ObjectContext.AttachSingleObject(IEntityWrapper wrappedEntity, EntitySet entitySet, String argumentName) 
at System.Data.Objects.ObjectContext.AttachTo(String entitySetName, Object entity) 
at System.Data.Entity.Internal.Linq.InternalSet`1.ActOnSet(Action action, EntityState newState, Object entity, String methodName) 
at System.Data.Entity.Internal.Linq.InternalSet`1.Attach(Object entity) 
at System.Data.Entity.DbSet`1.Attach(TEntity entity) 
at ... snip ...
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var myObject = db.MyObjectCollection.Find(x=>x.id = myId);


 var myCollection = db.MyObjectCollection.Where(x=>x.id = myId);
 if(myCollection != null && myCollection.Count() > 0){
     var myObject = myCollection.First();
 }

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