var ids = from prd in context.Positions.Where(p => p.order_id == order_id)
select new
{
prd.product_id
};var products = from prd in context.Products
where ids.Contains(prd.id)
select prd;
var products = from p in context.Products
join o in context.Orders on p.Order_Id equals o.Id
where o.Id == <НОМЕР ЗАКАЗА>
select p;