title: mongodb unwind array nested inside an array of documents
body: <p>In MongoDB, I need to be able to unwind nested an array in a document inside an array inside the main document.</p>
  
  <pre class="lang-json prettyprint-override"><code>{
      "_id" : ObjectId("5808d700536d1a3d69f4cf51"),
      "last_name" : "Maity",
      "xiith_mark" : 58,
      "id" : "3539488",
      "first_name" : "Harshavardhan",
      "course_name" : "BE/B.Tech",
      "institute_name_string" : "Abhayapuri College, P.O. Abhayapuri",
      "profile_percentage" : 45,
      "xiith_mark_type" : "Percentage",
      "xth_mark_type" : "Percentage",
      "date_of_birth" : "14-April-1993",
      "xth_mark" : 30,
      "last_login" : 1470827224,
      "percentage" : 55,
      "job_details" : [
          {
              "status" : NumberLong(6),
              "applied_date" : NumberLong(1470831441),
              "job_id" : NumberLong(92928),
              "contact_viwed_status" : 0,
              "label_name" : [
                  "shortlisted",
                  "rejected"
              ],
              "questionnaire_status" : 0,
              "batch_id" : NumberLong(6),
              "call_letter" : NumberLong(812)
          }, 
          {
              "status" : NumberLong(6),
              "applied_date" : NumberLong(1470831441),
              "job_id" : NumberLong(92928),
              "contact_viwed_status" : 0,
              "label_name" : [
                  "shortlisted",
                  "rejected"
              ],
              "questionnaire_status" : 0,
              "batch_id" : NumberLong(6),
              "call_letter" : NumberLong(812)
          }
      ],
      "branch_name" : "Applied Electronics",
      "candidate_state_name" : "West Bengal",
      "candidate_city_name_string" : "Kolkata",
      "10" : 10,
      "12" : 12,
      "skills" : "",
      "gender" : "Male",
      "fw_id" : "FW15884830",
      "cgpa" : 0,
      "picture_path" : "",
      "hq_passout_year" : 2019
  }
  </code></pre>
  
  <p>Based on the record above I need to count the job labels (job_details.label_name).</p>
  
  <p>I have tried the following query:</p>
  
  <pre class="lang-js prettyprint-override"><code>db.response.aggregate(
      {"$match":type_match},
      {"$unwind": "$job_details" }, 
      {"$group": 
        {
          "_id":"$job_details.label_name",
          "count": {"$sum": 1 }
        }
      }
  ])
  </code></pre>
  
  <p>The output is:</p>
  
  <pre class="lang-json prettyprint-override"><code>{
     "count": 2,
     "_id": [
     "shortlisted",
     "rejected"
      ]
  }
  </code></pre>
  
  <p>But I want the output to be:</p>
  
  <pre class="lang-json prettyprint-override"><code>[
    { 
        "count": 1,  
        "_id": "shortlisted"  
    },  
    {  
        "count": 1,  
        "_id": "rejected"  
    }
  ]
  </code></pre>
  
  <p>How can I get this output?</p>