Here's my domain model:

A User has many Transactions. A Portfoilo also has many Transactions. The relations are one to many and unidirectional. Traversing from Transaction to a User or Portfoilo is unnecessary. Transaction table has two foreign keys to each of the User and Porfoilo Table.

The problem is I am trying to make the mapping as IDBAG. (Since BAGs are not nice when inverse = false). My question is :

Is it safe to use the PK of Transaction table to use as a surrogate key for both of the User and Portfoilo IDBAG mappings ?

It seems reasonable.
Do you have problems with this solution?

Yes. The thing is PK is autogenerated by the Transaction Entity.

Do I have to use a generator from parent even the child generates its own PK and that PK used as a surrogate key ?

Can someone show me how to map basic idbag for complex object of parent and children ?