LINQ to SQL不会根据关系生成集合的原因是什么？

I have a relationship between two entities (e1 and e2) and e1 has a collection of e2, however I have a similar relationship set up between (e2 and e3), yet e2 does not contain a collection of e3's, any reason why this would happen? Anything I can post to make this easier to figure out?I have a relationship between two entities (e1