Aggregation Help

Working with Data
,
1

I am finding it difficult to output a document

{
  skillId: new ObjectId("61a914ac1155e2fb40e8d9c1"),
  test: [
    {
      skills:[
  {
    description: 'HTML presentation and formatting tags',
    _id: new ObjectId("61a914ac1155e2fb40e8d9c0")
  },
  {
    description: 'HTML layouts, using groups of elements together',
    _id: new ObjectId("61a914ac1155e2fb40e8d9c1")
  },
  {
    description: 'Advanced concepts',
    _id: new ObjectId("61a914ac1155e2fb40e8d9c2")
  
],
    }
  ],
}

//aggregation
 descriptions: {
                    $map: {
                        input: "$test.skills",
                        as: "skills",
                        in: {
                            $cond: [
                                {$eq: ["$$skills._id", "$skillId"]},
                                {$indexOfArray: ["$$skills", "$skillId"]},
                                null
                            ]
                        }
                    }
                },

I want a result that does the following thing it matches the Id of skillId with test.skills._id and return its description i wrote the above map but it doesn’t return anything null can someone suggest me proper way of query nested arrays

2

It is not clear what you want to achieve. It would be easier for us to understand if you supply the desired result that you want.

However, I see 2 major issues with your code.

1 - The array skills is within an object of the array test and you seem to process test as if it was an object.

You need to $map, $filter or $reduce the array test.

2 - You use $indexOfArray on the $map variable $$skills.

The variable $$skills represents 1 element of the input array. If you want to refer to the input array (which is wrong, see point 1) you need a single $ sign, just like you do with $skillId. The confusion might come from the fact that your code diverge from the habit of using plural for array and singular for an element of the array; like if the array is skills, each element is a skill.

3

Hi, steevej thank you for the response. I figured out how to get the desired output. I was looking for a document like this
image

The aggregation I wrote seems so messy

{
        $addFields: {
            idx: {
                $map: {
                    input: "$test.skills", as: "skill", in: {
                        index: {
                            $indexOfArray: ["$$skill._id", "$skillId",],
                        },
                    },
                },
            }, description: {
                $map: {
                    input: "$test.skills", as: "skill", in: {
                        des: "$$skill.description",
                    },
                },
            },
        },
    }, {
        $addFields: {
            index: {$arrayElemAt: ["$idx.index", 0]},
        },
    }, {
        $addFields: {
            descriptionzo: {
                $arrayElemAt: ["$description.des", 0],
            },
        },
    },
  {
    $project: {
      _id: 0,
      skillId: 1,
      "test.skills":1,
      description: {$arrayElemAt: ["$descriptionzo", '$index']},
    },
  },

can you please suggest me a more optimized way for achieving the description.
Thanks.

4

As already mentioned

How do you handle the test array? In your example, it only has 1 element. You should read about $reduce and $filter.

5

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