var q = from c in customer
        join o in order on o.fk_custId equals c.custId
        join ol in orderLines on ol.fk_orderId equals o.orderId
        join fi in foodItem on fi.foodId equals ol.fk_foodId
      \\  where 
        group new { c, o, ol, fi }  into g
        select new 
        { 
           orderId= o.orderId,
           date = o.date, 
           Total = g.Sum(x => x.fi.price* x.ol.quantity),
           TotalPrice= g.Sum(x => x.ol.extraPrice + x.m.Total )
        };

表连接不是特定的。请修改，因为我举例。请尝试此查询。

Table Joins are not specific. Please modify, as i was giving example. Please try this query.