[mongodb] Random record from MongoDB

In order to get a determinated number of random docs without duplicates:

  1. first get all ids
  2. get size of documents
  3. loop geting random index and skip duplicated

    number_of_docs=7
    db.collection('preguntas').find({},{_id:1}).toArray(function(err, arr) {
    count=arr.length
    idsram=[]
    rans=[]
    while(number_of_docs!=0){
        var R = Math.floor(Math.random() * count);
        if (rans.indexOf(R) > -1) {
         continue
          } else {           
                   ans.push(R)
                   idsram.push(arr[R]._id)
                   number_of_docs--
                    }
        }
    db.collection('preguntas').find({}).toArray(function(err1, doc1) {
                    if (err1) { console.log(err1); return;  }
                   res.send(doc1)
                });
            });
    

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