Xpath- constrains for reference selector

hi all,

i have a reference selector which i am calling via database. 

i need to get the list values  from a constrain related to the associated entity. 

for eg. i have an entity A( att1 as attribute)  and B ( att2 as attribute )

i need a constrain like this [att2= $A/att1 ] where i get list of B whose att2 have same values of that of att1 from B.

how do i achieve this ? 

(I am not getting right results when i use a association to pull it . Like [A_B/A/att1= att2]
hence i guess i should use '[% currentobject%]' as  A is the current object but not sure how)