EF用导航属性遍历从表时，删除主表出错

 var entitys= Repository.Table.Where(a => ids.Contains(a.UUID)).ToList();
 entitys.ForEach(a =>
 {
 　　foreach (var deductionitem in a.Deductionitems){}
 　　DoDelete(a);
}

DeductionItems和主表配了级联删除，代码执行时会抛The relationship could not be changed because one or more of the foreign-key properties is non-nullable

解决方法：

遍历子表后，EF无法分辨该记录是否要保留。所以需要手动删除从表，不能再依赖级联删除