mongoDB Array length on children element [duplicate] - arrays

Question

This question already has answers here:
MongoDB: count the number of items in an array
(3 answers)
Closed 5 years ago.
I have a data similar to the below in my mongo DB. Obviously the length of array1 is two and I am looking on a way to count the length of such array with mongo db script. Thanks
{
"_id" : ObjectId("65465465465465465468"),
"header" : {
"test" : "test",
},
"value" : 11.763548134011048,
"array1" : [
{
"q" : {
"value" : 250
}
},
{
"q" : {
"value" : 30,
}
}
],
"array2" : [ ]
}

Answer 1

You can use the $size aggregation operator.
Given the document you showed in your OP, the following command ...
db.collection.aggregate(
[
{
$project: {
sizeOfArray1: { $size: "$array1" }
}
}
]
)
... will return:
{
"_id" : ObjectId("65465465465465465468"),
"sizeOfArray1" : 2
}
This approach is valid for all versions of Mongo >= 2.6.
Update: in response to this comment ...
If I have more nested objects until I reach to my array I can use they same code and I just change the name of the field to the item that I am interested in?
You address any document attribute using path notation with a . to denote each 'level'. So, if your document looked like this:
{
"_id" : ObjectId("65465465465465465468"),
"header" : {
"test" : "test",
"array" : [1, 2, 3]
},
"value" : 11.763548134011048,
"array1" : [
{
"q" : {
"value" : 250
}
},
{
"q" : {
"value" : 30,
}
}
],
"array2" : [ ]
}
... then the following command would return the count of the array field inside the header nested sub document:
db.collection.aggregate(
[
{
$project: {
sizeOfHeaderArray: { $size: "$header.array" }
}
}
]
)
So, perhaps you can now see that you would address the array field inside the header nested sub document using header.array.
Hope that helps.