Use "Attach" as an Upsert in Entity Framework Core

Question

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.)

Answer 1

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; }

Answer 2

When you insert a row into one of your tables is the Id column of the database
table configured to automatically generate a new guid? using newid() or newsequentialid() ? The latter is preferable as it indexes better. Non sequential guids can impact performance in insert heavy tables.

Here is an example of a table definition that has a store generated Id column.

CREATE TABLE [dbo].[MyTable] (
    [Id] [uniqueidentifier] NOT NULL PRIMARY KEY CLUSTERED CONSTRAINT [PK_MyTable_Id] DEFAULT (newsequentialid()) 
)

The EF model would then be specified like so...

[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public Guid Id { get; set; }