MongoDB - Query to count the nested documents in an array - mongodb

Question

MongoDB - Layout
I'm a newbie to MongoDB, so pardon me if I'm using wrong terminologies here.
Given the above layout (attached image), I need to come up with two queries:
Use case 1 : A query that would output the count of total number of elements under someArrayField for a given document identifier.
For example, for the above layout, I expect the query to return 2 for someArrayField, because it has 2 elements under it as nested documents.
Use case 2 : A query that would output the count of total number of elements under someArrayField for every document under Collection_1 (Collection_1 can contain multiple documents with the similar layout).
I know, I need to somehow use "Aggregation Pipeline" to get the desired results, but due to the lack of experience with NoSQL, I'm struggling a bit here.
So, any help is greatly appreciated!

Answer 1

Given the following test set :
{
"_id" : ObjectId("5b453cc7799fb211dc44a1e8"),
"id" : 1.0,
"nestedDoc" : {
"nestedArray" : [
{
"field1" : "value1",
"field2" : "value2",
"field3" : "value3"
},
{
"field1" : "value11",
"field2" : "value21",
"field3" : "value31"
},
{
"field1" : "value12",
"field2" : "value22",
"field3" : "value32"
}
]
}
}
{
"_id" : ObjectId("5b453d23799fb211dc44a1e9"),
"id" : 2.0,
"nestedDoc" : {
"nestedArray" : [
{
"field1" : "value1",
"field2" : "value29",
"field3" : "value39"
},
{
"field1" : "value118",
"field2" : "value281",
"field3" : "value381",
"field4" : "value281"
},
{
"field1" : "value172",
"field2" : "value272",
"field3" : "value372"
}
]
}
}
Use case 1 :
Run the following aggregation query :
db.test1.aggregate(
[
// Stage 1 : filter documents
{
$match: {
id:1
}
},
// Stage 2 : count array size and return only it.
{
$project: {
arraySize:{$size:"$nestedDoc.nestedArray"},
_id:0
}
},
]
);
**Use case 2 ** Run the following :
db.test1.aggregate(
[
// Stage 1 count array size for each document
{
$project: {
_id:0,
arraySize:{$size:"$nestedDoc.nestedArray"}
}
},
// Stage 2 group all documents and sum arraySize
{
$group: {
_id:null,
totalArraysSize:{$sum:"$arraySize"}
}
},
]
);