I would like to get a sum from an array of arrays, but so far I have not found any solution.
The arrays look like this, and I would like to get the sum of all array values [2]
So with this array I should get the sum value of 13317 + 4719. I tried using $sum but I don't know how to point to the index 2 when it's inside of another array - $median_sale_prices.2 doesn't work in this case.
You may need to flatten the array field first by using the $unwind operator, after which you can then use the $arrayElemAt operator as an expression within the $sum accumulator operator to sum just the array element in a $group pipeline step.
The concept can be best explained with the following example:
Populate test collection
db.test.insert([
{
"x": [
[1, 1, 1],
[0, 1, 1],
[1, 0, 1],
[1, 1, 0],
[1, 0, 1],
[0, 1, 1]
]
}
])
Run aggregation query
db.test.aggregate([
{ "$unwind": "$x" },
{
"$group": {
"_id": null,
"total": {
"$sum": {
"$arrayElemAt": [ "$x", 2 ]
}
}
}
}
])
Sample Output
{
"_id" : null,
"total" : 5
}
Applying the above to your case becomes trivial.
$multiply: { parseInt($arrayElemAt: [ "$median_sale_prices", 1 ]), parseInt($arrayElemAt: [ "$median_sale_prices", 2 ]) }