ASP.NET MVC with Entity Framework - Update/Save complex type property

Question

I have 3 classes, related to 3 database tables:

public class Stat
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Quantity> Quantities { get; set; }
}
public class Quantity
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual Stat Stat { get; set; }
    public virtual Unit Unit { get; set; }
}
public class Unit
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Abbreviation { get; set; }
    public List<Quantity> Quantities { get; set; }
}

A Stat can have a list of Quantities, and a Quantity have one Unit. If I want to save a Stat with Entity Framework, I have to iterate through its list of Quantities, and check if the Quantity already exists, or newly created (the data comes from an HTML Form).

public void UpdateStat(Stat stat)
{
    foreach (Quantity q in stat.Quantities)
    {
        if (q.Id == 0)
        {
            db.Quantities.Add(q);
        }
        else
        {
            db.Entry(q).State = EntityState.Modified;
        }
    }
    db.Entry(stat).State = EntityState.Modified;
    db.SaveChanges();
}

The problem is, when more Quantities have the same Unit, an error occurs: "An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key."

I have no idea, how a Stat can be updated with the list of its Quantities and Units together.

Are you using code first? Does the DB have increment set on your Id columns? — Jay
– Jay, Commented Aug 3, 2013 at 20:03
Yes, I use code first, and the DB auto increments the IDs, but I didn't set anything... — Csati
– Csati, Commented Aug 3, 2013 at 20:12
Is stat attached to the context? If so you do not need to add the entities individually, simply having them in the Quantities collection of stat will apply the appropriate operation. — Jay
– Jay, Commented Aug 3, 2013 at 22:34
I use guids for my Id's so I don't use auto-inc too much, but what happens if you don't ever set the Id's on your model? His are all 0 if added, is EF smart enough to know that it's okay with auto-inc? — Tutan Ramen
– Tutan Ramen, Commented Aug 4, 2013 at 7:48

nhrobin · Accepted Answer · 2013-08-03 22:34:13Z

1

As you mentioned, the problem occurs when more than one quantity has same Unit. To avoid adding same unit multiple time you can check if a unit is already in the db. If it exists then reuse the existing unit.

if (q.Id == 0)
{
   var unit=db.Units.FirstOrDefault(u=>u.Id==q.Unit.Id);
   if(unit!=null)
      q.Unit=unit;
   db.Quantities.Add(q);
}

answered Aug 3, 2013 at 22:34

nhrobin

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1 Comment

Surya Deepak Over a year ago

using SingleOrDefault(u=>u.Id==q.Unit.Id) would be more appropriate,if there are two records with same id the FirstOrDefault would fetch those records and would give you the first item whereas singleOrDefault would only give you a single item and return null if there are no records.

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