Question 329227 in Javascript Question posted in
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Javascript – How to do a check on a specific element in an array property in mongodb

I’m trying to optimize my code that I’m currently running on a node server.
The part of the code that I’m trying to optimize is code related with filtering something called "content" part of it’s schema is

const contentSchema = new Schema({
title: String , 
skills :[
{
skillId:ObjectId , 
skillLevel:Int
}]
})

I’m trying to filter it by the difficulty. each content’s difficulty is based on the skill with the highest level. so if the skill with the highest level is 5 then the content difficulty is 5.
How I’m currently doing it.

const content = await Content.find({
...someOtherFilters
}).lean()
content = content.map((content)=> {
const difficulty = content.skills.reduce((acc , curr)=> {
if (!acc || curr.skillLevel > acc) return curr.skillLevel
return acc 
} , 0)
return {
...content, 
difficulty
}
}).filter(({difficulty})=> {
return difficulty === desiredDifficulty
})

How can I convert this into some mongodb query that I can then use like this

const contentCount = await Cotnent.countDocuments(contentQuery)
Tags:

3

Answers


  1. 0

    Mapping and filtering can be done using aggregate.

    By using $max you can find which element in the skills array has the highest skillLevel and use $arrayElemAt to pluck it from the array. You can then use $set to set the difficulty to the value of that skillLevel.

    With this new difficulty property you can use $match to find documents that match your desiredDifficulty criteria.

    Something like this I think might help:

    const content = await Content.aggregate([
  {
    $addFields: {
      difficulty: {
        $arrayElemAt: [
          "$skills",
          {
            $indexOfArray: [
              "$skills.skillLevel",
              {
                $max: "$skills.skillLevel"
              }
            ]
          }
        ]
      }
    }
  },
  {
    $set: {
      "difficulty": "$difficulty.skillLevel"
    }
  },
  {
    $match: {
      difficulty: desiredDifficulty
    }
  }
])

    See HERE for a working example.

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  2. 0

    The answer by @jQuenny is correct and can be simplified to :

    db.collection.aggregate([
  {$match: {$expr: {$eq: [{$max: "$skills.skillLevel"}, desiredDifficulty]}}}
])

    See how it works on the playground example

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  3. 0

    Instead of calculating the maximum difficulty on the fly, it would be best to store it as a property on the document, e.g. maxDifficulty. It should be updated whenever a new skill is added to the document. You will have very simple queries afterwards and you can support them by creating an index on the new property, e.g.:

    {
  maxDifficulty: 4
}

    For updating existing documents, you can use an aggregation pipeline similar to the following (mongoplayground – thank you @jQueeny for the documents in your playground):

    [
  {
    $set: {
      maxDifficulty: {
        $first: {
          $sortArray: {
            input: "$skills.skillLevel",
            sortBy: -1
          }
        }
      }
    }
  },
  {
    $merge: {
      // Define merge as required
    }
  } 
]

    Please note that documents that do not contain any skills will not receive a maxDifficulty property. You can add an $ifNull operator when setting the field to set the maxDifficulty to 0 if needed.

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