SQL trigger on insert table not work in MVC

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SQL

MVC5

I have a trigger on even insert for Item table:

create trigger tr_NextIDItem on Item
for insert
as
begin
    DECLARE @lastIDItem varchar(6)
    SET @lastIDItem  = (SELECT TOP 1 IDItem from Item order by IDItem desc)
    UPDATE Item set IDItem = dbo.func_NextIDItem (@lastIDItem,'IT_',6) where IDItem = ''
end

It's worked when i test on SQL query. But it's not work in asp.net MVC.

My code on ItemController:

context.Item.Add(_item);
context.SaveChanges();

How i can make it work now?