According to oneunicorn,
DbSet.Attach puts all entities in the graph into the Unchanged state. However, entities will be put in the Added state if they have store-generated keys (e.g. Identity column) and no key value has been set.
I'm having trouble with the "store-generated keys" part. My database was created in SMSS, and my table has an Id column of type uniqueidentifier, not null. It is the primary key. But how do I tell SMSS it should be store-generated? And will C# find out about that when I run an import? (In particular, I want to do tests on an in-memory database, if possible.)
Set the default value of the column using the alter table SQL statement:
alter table myTable alter column myIdColumn default newid()
On the c# side, decorate your EF class to indicate that the value for that column is auto-generated:
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
Guid myIdColumn { get; set; }