How to access array of array using MongoDB

Question

I have nested json array and want to fetch patient_ids whose code value is M18.1 using mongo db query.

   {"zip":"04903","specialty":"cardiac","number_of_patients":250,"data":[{"gender":"male","age_data":[{"age":1,"diagnostic_data":[{"standard":"ICD","version":"9","disease_info":[{"code":"I15","patient_ids":["101","102","103"]},{"code":"I11","patient_ids":["101","110"]}]},{"standard":"ICD","version":"10","disease_info":[{"code":"M18.1","patient_ids":["101","110","111","112","113"]},{"code":"M19.1","patient_ids":["101","110","111","112","113"]}]}]},{"age":3,"diagnostic_data":[{"standard":"ICD","version":"9","disease_info":[{"code":"I15","patient_ids":["101","102","103"]},{"code":"I11","patient_ids":["101","110"]}]},{"standard":"ICD","version":"10","disease_info":[{"code":"M18.1","patient_ids":["101","110","111","112","113"]},{"code":"M19.1","patient_ids":["101","110","111","112","113"]}]}]}]}]}

I have tried with aggregate,$elemMatch,$project and $filter, it's working with one level of array.

db.collectionnale.aggregate([
{
"$match" : {
       "data" : {      
               "$elemMatch" : {
                  "$and" : [
                     { "gender" : "male" }
                    ]
                    }
         }
   }
  },{
   "$project" : {
       "specialty":1,
       "data": {                  
          "$filter" : {
             "input" : "$data",
             "as" : "data",
             "cond" : { "$eq" : [ "$$data.gender", "male" ] }

             }
          }
       } 
}])

Chandan Singh Sirola · Accepted Answer · 2019-07-19 12:28:24Z

Querying nested array is not that easy so you can $unwind your arrays first by using aggregate and then apply your query.

db.collectionnale.aggregate([
  { $unwind: "$data" },
  { $unwind: "$data.age_data" },
  { $unwind: "$data.age_data.diagnostic_data" },
  { $unwind: "$data.age_data.diagnostic_data.disease_info" },
  {
    $match:
      { "data.age_data.diagnostic_data.disease_info.code": 'M18.1' }
  },
  {
    "$group": {
      "_id": 0,
      "patientIds": { "$push": "$data.age_data.diagnostic_data.disease_info.patient_ids" }
    }
  },
  {
    "$project": {
      "_id": 0,
      "patientIds": {
        "$reduce": {
          "input": "$patientIds",
          "initialValue": [],
          "in": { "$setUnion": ["$$value", "$$this"] }
        }
      }
    }
  }
])

Output:

 {
        "patientIds" : [ 
            "101", 
            "110", 
            "111", 
            "112", 
            "113"
        ]
    }

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