xPath Constraint for Objects

Hi all, I recently asked a question on this forum where I was advised to use an xPath constraint for the following problem: “ Currently, I have three objects: A, B and C. A has a one-to-many with both B and C. Similarly, B has a one-to-many relationship with C.  When creating an instance of B, I would like the user to be able to select C objects only from those associated with the same A.  Is there any way I can accomplish this using the reference selector widget? “ However, I am finding it challenging to write an xPath constraint that does exactly this. Any help would be appreciated.    EDIT: here is a visualisation of the example 
1 answers

Just click the Editbutton at the bottom, on the right of ‘Constraint by’ and select the (probably only available) path from B to A to C: (A=Company, B=Employee)

The difference between the xpath and this constraint menu, is that in the xpath, you have no B-object to compare to the C-objects. As opposed to the ‘Constraint by’ that starts at B.