how to select random documents with some conditions fulfilled in MongoDB

Basically I have documents in which I have on field called "Difficulty Level" and value of this filed is between 1 to 10 for each documents.

So, I have to select random 10 or 20 documents so that in randomly selected documents , atleast 1 document should be there for each difficulty level i.e. from 1 to 10. means there should atlease one document with "Difficulty level" : 1 ,"Difficulty level" : 2 ,"Difficulty level" : 3 ............."Difficulty level" : 10.

So, How can I select documents randomly with this condition fulfilled ?

Thanks

I tried $rand operator for selecting random documents but can't getting solution for that condition.

If I've understood correctly you can try something like this:

The goal here is to create a query like this example

This query gets two random elements using $sample, one for level1 and another for level2. And using $facet you can get multiple results.

db.collection.aggregate([
  {
    "$facet": {
      "difficulty_level_1": [
        {
          "$match": { "difficulty_level": 1 } },
        { "$sample": { "size": 1 } }
      ],
      "difficulty_level_2": [
        { "$match": { "difficulty_level": 2 } },
        { "$sample": { "size": 1 } } 
      ]
    }
  }
])

So the point is to do this query in a dynamic way. So you can use JS to create the object query an pass it to the mongo call.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const random = Math.floor((Math.random()*10)+1) // Or wathever to get the random number

let query = {"$facet":{}}
for(let i = 1 ; i <= random; i++){
  const difficulty_level = `difficulty_level_${i}`
  query["$facet"][difficulty_level] = [
      { $match: { difficulty_level: i }},
      { $sample: { size: 1 }}
    ]
}
console.log(query) // This output can be used in mongoplayground and it works!
// To use the query you can use somethiing like this (or other way you call the DB) 
this.db.aggregate([query])

<!-- end snippet -->