M-N mappings with attributes or batch fetch

Question

There is one very common design case of M-N relationship with attributes.
Difference is that M-N is implemented by another persistent class with bunch
of attributes rather then by internal O/R mapper means.
For example we have
a Member who can be assigned to an OrgUnit (M-N) but assignment should carry
additional info Role, Date of assignment etc.
Member----Position----OrgUnit
In absence of specialized mapping for this case (TopLink, ObjectFrontier and
some other O/R frameworks have it) it is common practise to use two 1-N
Member----Position + OrgUnit----Position
The problem is performance of course - getting all positions for a
particular Member is fast (one SQL statement) but getting OrgUnit for each
fetched position is expensive (SQL call per fetched position if OrgUnit has
not been fetched yet).
It would be good to either define M-N mapping with attributes concept or at
least provide O/R mapper with a hint to do batch load of OrgUnits when
reading Positions for a Member (and vice versa Members when loading
Positions for an OrgUnit) by joining OrgUnit table with Position table and
loading it together

Answer 1

Hi Abhijeet,
i think i should have mentioned this earlier. My Employee node is inside another node.  So, the actual input structure of the WS is this:
InputValues (1...1)
   Employees (0..n)
      FirstName: String
      LastName: String
This scenario works OK in BPM when mapping a 0..n node using deep copy as Jocelyn explained, but I still doesn't work if I want to pass an empty (not null) Employees array.
If I go to the WS Navigator and run this WS with the following parameters, it runs ok (I get an output message saying no employee was selected, which is how it should work):
InputValues -  "Is null" checkbox not activated.
   Employees - "Skip" checkbox not activated
      FirstName - "Skip" checkbox activated
      LastName - "Skip" checkbox activated
However, if instead of activating the checkbox of, say, "FirstName", I enter a "" value, I get an error from the WS saying that's not a vaild first name, which is also how it should work.
In java code, I would just pass an empty InputValues object to the WS, but I'm not sure how to do this in a BPM without it being considered null, and without having to set on of its String-child values to "".
Do you know how to achieve this?