Query:

dbCollection.findOne(
      { aId: a, 'pi.Id' : b },
      { "pi.$" : 1  },
      function(err, result) {}
)

Result with v2:

{ _id: 5a9e8594c926462d20f1f14f,
  pi:
   [ { Id: 5a9e85a9f2b3052aa8fd7d37,
       	Uid: 5a0db4b5409339cc35ff4f38 } ] 
}


Result with v3

{ _id: 5a9e8594c926462d20f1f14f,
  aId: 5a9e8594c926462d20f1f14e,
  p:
   [ { Id: 5a9e859412e41f2d20b888a3,
       Tid: 000000000000000000000001},
     { Id: 5a9e85a9f2b3052aa8fd7d37,
       Tid: 5a96ce19ca25c326f8306900}],
  pi:
   [ { Id: 5a9e859412e41f2d20b888a3,
       Uid: 000000000000000000000001 },
     { Id: 5a9e85a9f2b3052aa8fd7d37,
       Uid: 5a0db4b5409339cc35ff4f38 } ],
}