This thread looks to be a little on the old side and therefore may no longer be relevant. Please see if there is a newer thread on the subject and ensure you're using the most recent build of any software if your question regards a particular product. Otherwise - post away!
Consider a model where a Contribution has a nullable ApprovedBy association to Member, and Member has the corresponding Approvals reverse association. Then:
// Returns all Contributions where the ApprovedBy association
// Returns all Members where the Approvals collection contains