现在查询的需求是查出所有typeof(NoteCategory)的对象，满足Parents.Count = 0的条件；
但是发现使用ObjectPath "Parents.Count=0"或"Parents[Count]=0"都无法进行查询，还抛出了异常；于是就到了Base4.Net的Forums上去问，结果得到的答案是：

Ask (Me) :

Answer: (Alex James)

I don't support embedding a count filter on a relationship in objectpath yet... sorry.

Although what you are asking for should be added... I will have a think about how in the next few days.
The problem is that I collapse calls to object.Relationship so that you are querying the related object not the actual relationship table.

What needs to be done is to add additional HardCoded filters to the object.Relationship so that you can do the sort of things you are asking for....I will probably add support for this before Beta3.

While you can't do it with object path you can do it...

If you use an ObjectCommand and write the SQL yourself.
Execute the ObjectCommand so it returns a reader (ExecuteReader) and contruct an ObjectDataReader around the returned reader so you can read your NoteCategory objects.

It depends how you defined your RelationshipDef, if Parent is defined with LinkIsSource and a RelationshipName = Parent
This would return you all NoteCategories without parents:
SELECT * FROM [NOTECATEGORY] WHERE ID NOT IN (SELECT  DISTINCT(Source) FROM [Maxtiviti.Storage.Relationship] WHERE Name = 'Parent')

When I get a chance I will post you a snippet on how to use this code with a ObjectCommand...

呵呵，真希望alex快出beta3版啊，要不我用以下的解决方案，有很大的性能问题啊：