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• $map to iterate loop of subRoom array and in to return required fields
• $filter to iterate loop of notifications and $size to get total elements from filtered result

Add new stage after group stage,

• $range to create an array of numbers from 0 to length of kcList array using $size
• $map to iterate loop of the above range
• $arrayElemAt to get element of range position in kcList array
• $multiply to multiply range number with kcList value returned from the above operation

Each array filter should be its own object in the array, like

I want to know if there is any solution avoiding aggregation.

There is no other option except aggregation, To achieve your expected result you must have to use Aggregation Array Expression Operators.

If you really want to use find methods then you can use Aggregation Expression starting from MongoDB 4.4,

Starting in MongoDB 4.4, as part of making find() and findAndModify() projection consistent with aggregation's $project stage,
The find() and findAndModify() projection can accept aggregation expressions and syntax.
MongoDB enforces additional restrictions with regards to projections. See Projection Restrictions for details.

do u know if using aggregation operator inside find projection is faster or slower (or equal :) ) in terms of performance?

As per find-projection-documentation there is no any specification about performance, but i think both are equal.

If you really want to check the performance of both methods then you can check by explain() executionStats mode.

• query from tolls collection
• $project to show required fields and add new field type for "tolls"
• $unionWith with fuel collection and $project to show required fields add new field type for "fuel"
• now we have merged both collections document in root and added type for each document
• $group by date and type, count total elements
• $group by only date and construct the array of both type with its count in key-value format
• $addFields to convert that analytic field to object using $arrayToObject

What you have tried is so much appreciated. But in $project you need to use $reduce which helps to loop through the array and satisfy the condition

Here is the code

I have explained your previous problem in Question. Since the invoiceInfo is an array and invoiceData also an array inside invoiceInfo, we use map and filter. Then we need to exclude empty array of invoiceData. (This can be done in previous step also like filter-map->filter, but it could be lengthy, thats why I used it in next stage )

Here is the code

• $map to iterate loop of subDoc array
• $sum to get total of subArray array of numbers
• $ifNull to check if field is not present or null then return empty array because $size operator only allows array input
• $size to get total elements in subArray array
• $mergeObjects to merge current object with new added fields

Since each post have multiple comments, after unwinding comments you need to group it together to form an array

Update I have updated the fetch approach a lil bit like the below.