I'm using Play 2.3.7, Mongo 2.4.8 and reactivemongo 0.10.5 and I've got some surprising things...
When start the shell and run :
db.Exponents.aggregate([
{$match: {eventId: {$in: ["3dac7440-f8e5-497d-8b0b-77127d16e1ae", "fcb07f0c-2ff3-4408-b83e-8f42d6a00112"]}}},
{$group: {_id: "$eventId", eventCount: {$sum: 1}}}
])