How to find distinct values of fields using facet operation in mongodb

Question

The filteredAccording part and the categorizedBy is working as expected using the query which I provided in the link but I am facing issues in the findDistinct part.

In mongodb I have the following data:

 {
        "_id": 10001,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "ACTIVE",
        "isMarked": true
      },
      {
        "_id": 10002,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "NON-ACTIVE",
        "isMarked": true
      }

I wanted the response to be:

 "university": [
  {
    "name": "Literature",
    "values": [
      {
        "_id": 10001,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "ACTIVE",
        "isMarked": true
      },
      {
        "_id": 10002,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "NON-ACTIVE",
        "isMarked": true
      }
    ]
  }
],
 "findDistinct": [
    {​​​​​​​​
      "name": "Courses",
      "values": [
        "English",
         "French"
      ]
    }​​​​​​​​,
    {​​​​​​​​
      "name": "Status",
      "values": [
        "ACTIVE",
        "NON-ACTIVE"
      ]
    }​​​​​​​​
  ]

I tried it using this link but the response is not coming as expected. https://mongoplayground.net/p/XECZvRMmt3T

Right now, the response is coming like this

 "universities": [
  {
    "name": "Literature",
    "values": [
      {
        "_id": 10001,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "ACTIVE",
        "isMarked": true
      },
      {
        "_id": 10002,
        "university": "SPYU",
        "Courses": [
          "English",
          "French"
        ],
        "dept": [
          "Literature"
        ],
        "type": [
          "Autonomous"
        ],
        "status": "NON-ACTIVE",
        "isMarked": true
      }
    ]
  }
],
"findDistinct": [
    {​​​​​​​​
      "Courses": [
        "English",
         "French"
      ]
    }​​​​​​​​,
    {​​​​​​​​
      "status": [
        "ACTIVE",
        "NON-ACTIVE"
      ]
    }​​​​​​​​
  ]

Any Help will be appreciated!!

Answer 1

Quick fixes in your query,

universities:

• $addFields, remove $project and add only one operation for isMarked
• $unwind deconstruct dept array
• $group by dept and get values array of root

findDistinct:

• $group by null and get unique courses array and status
• $reduce to iterate loop of Courses nested array and get unique array using $setUnion
• Make array of course and status in dest field
• $unwind deconstruct dest array
• $replaceRoot replace dest object to root

db.collection.aggregate([
  { $match: { university: "SPYU" }
  },
  {
    $facet: {
      universities: [
        { $addFields: { isMarked: { $in: ["French", "$Courses"] } } },
        { $unwind: "$dept" },
        {
          $group: {
            _id: "$dept",
            values: { $push: "$$ROOT" }
          }
        }
      ],
      findDistinct: [
        {
          $group: {
            _id: null,
            Courses: { $addToSet: "$Courses" },
            Status: { $addToSet: "$status" }
          }
        },
        {
          $project: {
            _id: 0,
            dist: [
              {
                name: "Courses",
                values: {
                  $reduce: {
                    input: "$Courses",
                    initialValue: [],
                    in: { $setUnion: ["$$this", "$$value"] }
                  }
                }
              },
              {
                name: "Status",
                values: "$Status"
              }
            ]
          }
        },
        { $unwind: "$dist" },
        { $replaceRoot: { newRoot: "$dist" } }
      ]
    }
  }
])

Playground