EF连接有效但SelectMany没有

Question

为什么这样做：

// find all second level categories
from c in Categories
where c.ParentId == null
join c2 in Categories on c.Id equals c2.ParentId
select c2

但是以下引发 System.NotSupportedException：无法创建类型为“Category”的常量值。在此上下文中仅支持原始类型（例如Int32，String和Guid'）：

from c in Categories
where c.ParentId == null
from c2 in Categories
where c.Id == c2.ParentId
select c2

注意：我真正想做的是使用包含like的连接条件将表连接到自身：

from c in Categories
from c2 in Categories
where c.Lineage like c2.Lineage + '%'
select c

Answer 1

怎么样

from c in Categories
from c2 in Categories
where c.Lineage.StartsWith(c2.Lineage)
select c