[{"_id":"5f969419d40c1580f2d4aa31","users":{"foo@bar.com":"baz"}},{"_id":"5f9694d4d40c1580f2d4aa33","users":{"baz@test.com":"foo"}}]

db.collection.aggregate([{"$project":{"users":{"$objectToArray":"$users"}}},{"$match":{"users.k":"foo@bar.com"}},{"$project":{"users":{"$arrayToObject":"$users"}}}])