FIND SPECIFIC FIELDS FROM SunDOCUMENTS OF ALL DOCUMENTS UNDER A COLLECTION IN MONGODB
//use when you want `_id,size` (As object) with its only property `h` in
//$project,
db.inventory.aggregate([{$match:{ item: /^j/ } },{ $project:{
"size.h":"$size.h" } }])
//use when you want `_id,size` size ase whole object
//$project,
db.inventory.aggregate([{$match:{ item: /^j/ } },{ $project:{ "size":1 }
}])