mongodb - Filter the sub array of an array by some criteria

Question

Hi have a document in the format :

{
   "_id":"someId",
   "someArray":[
      {
         "subId":1,
         "subArray":[
            {
               "field1":"A",
               "filterMe":"NO"
            },
            {
               "field1":"B",
               "filterMe":"YES"
            }
         ]
      },
      {
         "subId":2,
         "subArray":[
            {
               "field1":"C",
               "filterMe":"YES"
            },
            {
               "field1":"D",
               "filterMe":"NO"
            }
         ]
      }
   ]
}

how can i filter the subArray based on some criteria. Example filter out the subArray if field filterMe is "YES". so finally the output json should be like

{
   "_id":"someId",
   "someArray":[
      {
         "subId":1,
         "subArray":[
            {
               "field1":"A",
               "filterMe":"NO"
            }
         ]
      },
      {
         "subId":2,
         "subArray":[
            {
               "field1":"D",
               "filterMe":"NO"
            }
         ]
      }
   ]
}

I tried in the below way but, its filtering the whole subArray.

db.testJson.aggregate([
    { $project: {
        'someArray.subArray': {
          $filter: {
            input: '$someArray.subArray',
            as: 'input',
            cond: {$eq: ["$$input.filterMe", "YES"]}
        }}
    }}
]);

Answer 1

You just need a $filter inside a $map:

db.junk.aggregate([
  { "$project": {
    "someArray": {
      "$filter": {
        "input": {
          "$map": {
            "input": "$someArray",
             "as": "some",
             "in": {
               "subId": "$$some.subId",
               "subArray": {
                 "$filter": {
                   "input": "$$some.subArray",
                   "as": "sub",
                   "cond": { "$ne": [ "$$sub.filterMe", "YES" ] }
                 }
               }
             }
          }
        },
        "as": "some",
        "cond": { "$gt": [ { "$size": "$$some.subArray" }, 0 ] }
      }
    }
  }}
])

Produces:

{
        "_id" : "someId",
        "someArray" : [
                {
                        "subId" : 1,
                        "subArray" : [
                                {
                                        "field1" : "A",
                                        "filterMe" : "NO"
                                }
                        ]
                },
                {
                        "subId" : 2,
                        "subArray" : [
                                {
                                        "field1" : "D",
                                        "filterMe" : "NO"
                                }
                        ]
                }
        ]
}

I actually wrap that in an additional $filter to remove any someArray entries where the filtered subArray ended up being empty as a result. Mileage may vary on that being what you want to do.