Here's a more fail-proof method which will do the trick without much fuss:
Step#1: Place this piece of code right above the migration you want to debug:
public partial class ORACLE_Test : DbMigration
{
public override void Up()
{
if (!System.Diagnostics.Debugger.IsAttached)
System.Diagnostics.Debugger.Launch();
AddColumn("TEST", "UR_USER_ID", x => x.Decimal(nullable: false, precision: 11, scale: 0, storeType: "number"));
AddColumn("TEST", "UR_CLIENT_ID", x => x.Decimal(nullable: false, precision: 11, scale: 0, storeType: "number"));
[...]
}
public override void Down()
{
}
}
Step#2: Compile the project containing your migrations
Step#3: Open a console inside the output directory (/bin/Debug, /bin/Release etc) containing the dll of your migrations
Step#4: Invoke migrate.exe with the /scriptFile parameter to launch the debugger and actually debug the desired db-migration
migrate.exe "Your.Migrations.Assembly.dll" /scriptFile="foo.sql" /verbose /startupConfigurationFile="Your.Migrations.Assembly.config"
Once the debugger-selector dialog pops up pick the visual studio instance that you have already opened.