I need to filter a multi-level nested array in MongoDB. The schema is as follows,

    {
      "_id": "1234",
      "array1": [
        {
          "id": "a11",
          "array2": [
            {
              "id": "a21",
              "array3": [
                {
                  "id": "a31",
                  "status": "done"
                },
                {
                  "id": "a32",
                  "status": "pending"
                }
              ]
            }
          ]
        }
      ]
    }

The required output must filter array3 with condition status=done. Which is the best possible method to achieve the same?

CodePudding user response：

Use $map to iterate array1 and array2, use $filter to filter array3. Finally compare array3 with empty array for document matching.

db.collection.aggregate([
  {
    "$addFields": {
      "array1": {
        "$map": {
          "input": "$array1",
          "as": "a1",
          "in": {
            id: "$$a1.id",
            array2: {
              "$map": {
                "input": "$$a1.array2",
                "as": "a2",
                "in": {
                  id: "$$a2.id",
                  array3: {
                    "$filter": {
                      "input": "$$a2.array3",
                      "as": "a3",
                      "cond": {
                        $eq: [
                          "$$a3.status",
                          "done"
                        ]
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  {
    $match: {
      "array1.array2.array3": {
        $ne: []
      }
    }
  }
])

Here is the Mongo playground for your reference.

CodePudding user response：

Query

• almost the same as @ray query
• $mergeObjects allows us to not write the fields by hand($setField can be used also if mongoDB5 ) (if instead of id you also have 10 fields, this will work without changing the query)

Playmongo

aggregate(
[{"$set": 
   {"array1": 
     {"$map": 
       {"input": "$array1",
        "as": "a1",
        "in": 
         {"$mergeObjects": 
           ["$$a1",
             {"array2": 
               {"$map": 
                 {"input": "$$a1.array2",
                  "as": "a2",
                  "in": 
                   {"$mergeObjects": 
                     ["$$a2",
                      {"array3": 
                       {"$filter": 
                        {"input": "$$a2.array3",
                         "as": "a3",
                         "cond": {"$eq": ["$$a3.status", "done"]}}}}]}}}}]}}}}}])