[{"collection":"collection","count":10,"content":{"_id":{"type":"enum","values":[0,1,2,3,4,5,6,7,8,9]},"array_field":{"type":"array","arrayContent":{"type":"object","objectContent":{/** required, list of generator used to*/"sub_field":{"type":"enum","values":["a","b","c","d"]}}}}},"indexes":[{"name":"ix","key":{"_id":1,"array_field.sub_field":1}}]}]

db.collection.aggregate([{$match:{"_id":{$in:[0,1,2,3,4,5,6,7,8,9]}}},{$unwind:"$array_field"},{$match:{"array_field.sub_field":"a"}}]).explain("executionStats")