i am having a problem in hibernate. i have an object graph say as follows :

A__
|
| (collection of B)
|__
B

B's collection id cascaded "all" on A object.

i fetch an object of A by id and update its primitive fields. but when i call update method of session for updating A, hibernate fires update query for B's objects even though i have not made them dirty. and another main thing is that i am finding and updating objects using different sessions.

one solution that i could look for is use select-before-update attribute of class tag (for using for B class) but that would be too costly for this purpose.
Isn't there any way hibernate track this dirty checking?

No.
Unless you implement a smart interceptor