$unwind does not work when there is an array inside an object. i.e { $unwind: "$a.sizes" }

[felipecarrillo100]

$unwind does not work when the array is inside an object. For instance if the array is inside object a, as in the example below:
var data = [
{ "_id" : 1, "item" : "ABC", "a":{"sizes": [ "S", "M", "L"] }},
{ "_id" : 2, "item" : "EFG", "a":{"sizes" : [ ] }},
{ "_id" : 3, "item" : "IJK", "a":{"sizes": "M"} },
{ "_id" : 4, "item" : "LMN", "a":{}},
{ "_id" : 5, "item" : "XYZ", "a":{"sizes" : null} }
]

            var a = mingo.aggregate(data, [ { $unwind: "$a.sizes" } ] )