[{"_id":1,donations:[{_id:"d1a",assigned:false},{_id:"d1b",assigned:false}]},{"_id":2,donations:[{_id:"d2a",assigned:false},{_id:"d2b",assigned:false}]},]

db.collection.update({_id:3,"donations._id":"d1b"},{$set:{"donations.$.assigned":true}})