I have a particular field in my document which has a multilevel nested array structure. The document looks like something this

{
    "_id" : ObjectId("62171b4207476091a17f595f"),
    "data" : [ 
        {
            "id" : "1",
            "content" : [ 
                {
                    "id" : "1.1",
                    "content" : []
                }, 
                {
                    "id" : "1.2",
                    "content" : [ 
                        {
                            "id" : "1.2.1",
                            "content" : [ 
                                {
                                    "id" : "1.2.1.1",
                                    "content" : []
                                }
                            ]
                        }, 
                        {
                            "id" : "1.2.2",
                            "content" : []
                        }
                    ]
                }
            ]
        }
    ]
}

(The ids in my actual data is a random string, I have added a more defined id here just for readability)

In my application code the nesting level is controlled so it won't go more than 5 levels deep.

I need to project a particular object inside one of the deeply nested arrays.

I have all the information needed to traverse the nested structure. For example if I need to fetch the object with id "1.2.2" my input will look something like this:

[{id: 1, index: 0}, {id: 1.2, index: 1}, {id: 1.2.2, index: 1}]

In the above array, each element represents one level of nesting. I have the Id and the index. So in the above example, I know I first need to travel to index 0 at the top level, then inside that to index 1 , then inside that to index 1 again to find my object.

Is there a way I can only get the inner object that I want directly using a query. Or will I need to get the whole "data" field and do the traversal in my application code. I have been unable to figure out any way to construct a query that would satisfy my need.

CodePudding user response：

Query

• if you know the path, you can do it using a series of nested
  • $getField
  • $arrayElemAt
• you can do it in one stage with nested calls, or with many new fields like i did bellow, or with mongodb variables

*i am not sure what output you need, this goes inside to get the 2 using the indexes (if this is not what you need add if you can the expected output)

Test code here

Data

[
  {
    "_id": ObjectId( "62171b4207476091a17f595f"),
    "data": [
      {
        "id": "1",
        "content": [
          {
            "id": "1.1",
            "content": []
          },
          {
            "id": "1.2",
            "content": [
              {
                "id": "1.2.1",
                "content": [
                  {
                    "id": "1.2.1.1",
                    "content": []
                  }
                ]
              },
              {
                "id": "1.2.2",
                "content": [1,2]
              }
            ]
          }
        ]
      }
    ]
  }
]

Query

aggregate(
[{"$set":
  {"c1":
   {"$getField":
    {"field":"content", "input":{"$arrayElemAt":["$data", 0]}}}}},
 {"$set":
  {"c2":
   {"$getField":
    {"field":"content", "input":{"$arrayElemAt":["$c1", 1]}}}}},
 {"$set":
  {"c3": 
   {"$getField":
    {"field":"content", "input":{"$arrayElemAt":["$c2", 1]}}}}},
 {"$project":{"_id":0, "c4":{"$arrayElemAt":["$c3", 1]}}}])

Results

[{
  "c4": 2
}]