count inner array mongoose - arrays

Question

I want to do a count on an array. This my model
modelDetail.aggregate([
{
$group: {
_id: '$main_section.dept',
count: {$sum: 'first_array.second_array.main_data'}
}
}
], function (err, result) {
if (err) {
//next(err);
console.log(err);
} else {
console.log(JSON.stringify(result));
res.json(result);
}
});
This is bringing out 0 for all the result. Output is below
[{"_id":"design","count":0},
{"_id":"training","count":0},
{"_id":"forecast","count":0},
{"_id":"internal audit","count":0},
{"_id":"research","count":0}]
I wanted to get the count of all the data
this is my schema
var userSchema = mongoose.Schema({
main_section :{
dept : String
},
first_array :[{//
floor : String,
second_array :[{//3rd level
name : String,//4th level
date : String
}]
}]
});
this is the data in the json
{ "_id" : "5bb39baf40f87f17f01734f8",
"main_section" : {
"dept" : "design"
},
"first_array" : [{ "_id" : "5bb39baf40f87f17f01734fc", "floor" : "2nd floor" ,
"second_array" : [
{ "_id" : "5bc102f6dff41a0e844dd2b7", "name" : "Blake Tyson", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc102fddff41a0e844dd2bb", "name" : "Meagan Shawn", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc1044dbba8b31ed42e7408", "name" : "Stephen Stone", "date" : "2018-10-12T00:00:00Z" }
]
}]
},
{ "_id" : "5bb39baf40f87f17f01734f8",
"main_section" : {
"dept" : "training"
},
"first_array" : [{ "_id" : "5bb39baf40f87f17f01734fc", "floor" : "1st floor",
"second_array" : [
{ "_id" : "5bc102f6dff41a0e844dd2b7", "name" : "Micheal Harrison", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc102fddff41a0e844dd2bb", "name" : "Favour Reality", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc1044dbba8b31ed42e7108", "name" : "Gift Myers", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc1044dbba8b31ed42e71a1", "name" : "Drake Hills", "date" : "2018-10-12T00:00:00Z" },
{ "_id" : "5bc1044dbba8b31ed42e74c2", "name" : "Hashtan Priest", "date" : "2018-10-12T00:00:00Z" }
]
}]
},
{ "_id" : "5b98987f08925c0f5cd86780",
"main_section" : {
"dept" : "forecast"
},
"first_array" : [{ "_id" : "5b98987f08925c0f5cd86784", "floor" : "4th floor",
"second_array" : [ ]
}]
},
{ "_id" : "5b98187f08924c0f5ad86790",
"main_section" : {
"dept" : "internal audit"
},
"first_array" : [{ "_id" : "5b98987f08925c0f5cd86784", "floor" : "4th floor",
"second_array" : [ ]
}]
},
{ "_id" : "5b98187f08924c0f5ad86790",
"main_section" : {
"dept" : "research"
},
"first_array" : [{ "_id" : "5b98987f08925c0f5cd86784", "floor" : "4th floor",
"second_array" : [ ]
}]
}
Basically what i was hoping to do is to let each of the array produces the total number of elements
in the array for second alongside with the main section so for example
design would output 3
training would output 5
forecast would output 0
internal audit would output 0
research would output 0
So, any help is greatly appreciated!