Using sum in Mongo - mongodb

Question

I'm learning mongo and I have this schema below and i would like some help defining a query:
I would like to get the sum of the "entregado" fields that match this code: 151001. In this case i would get this result = 38.
Do I need to change the schema or is easy to get a query for what i want?
{
"_id": 101,
"torre": 1,
"standard": {
"mamposteria": [
{
"codigo": 311017,
"descripcion": "LADRILLO ARCILLA H-10",
"cantidad": 1080,
"um": "UN",
"entregado": 1080,
"fecha": new Date('June 10, 2013'),
"vale": [1322]
},
{
"codigo": 311021,
"descripcion": "LADRILLO ARCILLA H-7",
"cantidad": 200,
"um": "UN",
"entregado": 200,
"fecha": new Date('June 10, 2013'),
"vale": [1322]
},
{
"codigo": 151001,
"descripcion": "CEMENTO GRIS 50 KG",
"cantidad": 17,
"um": "KG",
"entregado": 17,
"fecha": new Date('June 10, 2013'),
"vale": [1322]
}
],
"mortero": [
// . . .
],
"estructura":[
// . . .
]
}
}

Answer 1

To get the count alone u don't need to modify your schema.
Just following the below steps can give you the count what u need. I will not give you the whole query to find the count. That makes u lazy..so will give steps to get the count...
1) use $unwind 2) use $match to find the value for code: 151001 3) use $group to $sum the values present in entregado
This should give you the count.
If you need any other help pls contact me I will help