Database

[{array:[{date:"27-08-12",nestedArray:[0,1,2]},{date:"18-08-12",nestedArray:[0,2]}]}]

Query

db.collection.aggregate([{"$unwind":"$array"},{"$group":{"_id":null,"data":{"$push":{k:"$array.date",v:"$array.nestedArray"}}}},{"$project":{"array":{"$arrayToObject":"$data"}}}])

Result