Question: I am using Entity Framework 1 with .net 3.5.

I am doing something simple like this:

```
var roomDetails = context.Rooms.ToList();

foreach (var room in roomDetails)
{        
   room.LastUpdated = DateTime.Now;
}

```

I am getting this error when I try to do:

```
 context.SaveChanges();

```

I get the error:

> 
> **Unable to update the EntitySet - because it has a DefiningQuery and no <UpdateFunction> element exists in the <ModificationFunctionMapping> element to support the current operation.**
> 
> 
> 

I am doing lots of updates on the context and not having any issues, it's only when I try to update this particular entity.

All my searching shows up the same thing, that there is no primary key declared on the entity that I'm trying to update. But alas, I do have a Primary key declared...

Answer: 
This can also happen if data model is out of date.

Hopefully this will save someone else frustration :)
The error may have occurred if your table doesn't have a primary key, in this case the table is "read only", and the `db.SaveChanges ()` command will always error.