How do I use the results of $facet sub-pipelines

Working with Data
#1

I have a mongodb query that looks like the following:

[
    {$facet: {
        subpipeline1: [
          {$match: { ...
            // mongo subpipeline 1 query
          }  
        ]
        , subpipeline2: [
          {$match: { ...
             // mongo subpipeline 2 query
          }   
        ]
    }}
]

result of subpipeline1 is:

_id	            qty
category_1	    qty_1

result of subpipeline2 is:

_id	            qty
category_all	qty_all

I want to divide qty_1 (qty from subpipeline1) by qty_all (qty from subpipeline2)

Seeking the help of the community to guide me please.

#2

Results of each subpipeline will be array of documents, if you can give sample data of the results in JSON, and what is the expected output you need. Maybe $map can help you. But if you give sample data it will be helpful alot.

#3

Hi Takis, many thanks for your attention to my question. I intend to do the following please -

The result of subpipeline1 is the following

[
  {_id: 'category_1', qty: '10'},
  {_id: 'category_2', qty: '15'},
  {_id: 'category_3', qty: '35'},
  {_id: 'category_4', qty: '50'}
]

The result of subpipeline2 is the following

[
  {_id: 'category_all', qty: '110'}
]

Combining the above 2, I intend to get the following output:

[
  {_id: 'category_1', qty: '10', total: '110'},
  {_id: 'category_2', qty: '15', total: '110'},
  {_id: 'category_3', qty: '35', total: '110'},
  {_id: 'category_4', qty: '50', total: '110'}
]

Many thanks for your guidance.

#4

Data (after the $facet)

[
  {
    "subpipeline1": [
      {
        "_id": "category_1",
        "qty": "10"
      },
      {
        "_id": "category_2",
        "qty": "15"
      },
      {
        "_id": "category_3",
        "qty": "35"
      },
      {
        "_id": "category_4",
        "qty": "50"
      }
    ],
    "subpipeline2": [
      {
        "_id": "category_all",
        "qty": "110"
      }
    ]
  }
]

Query

aggregate(
[{"$unwind": {"path": "$subpipeline1"}},
 {"$project": 
   {"_id": "$subpipeline1._id",
     "qty": "$subpipeline1.qty",
     "total": {"$arrayElemAt": ["$subpipeline2.qty", 0]}}}])

Data out

[{
  "_id": "category_1",
  "qty": "10",
  "total": "110"
},
{
  "_id": "category_2",
  "qty": "15",
  "total": "110"
},
{
  "_id": "category_3",
  "qty": "35",
  "total": "110"
},
{
  "_id": "category_4",
  "qty": "50",
  "total": "110"
}]