Mongodb aggregation pass argument to element size of $sample - mongodb

Question

Hello every body here any one can help me with query below
I want to get quiz list with random amount
the amount of rendom will
base on each lesson
The problem is
mongodb not allow to pass argument to element size of $sample
Any one can give me the solution
lessonModel.aggregate([
{ $match : {'quiz.status':1 } },
{
$lookup : {
from : 'quiz',
let : { 'lesson_id' : '$_id','limit' : '$quiz.amount' },
pipeline : [
{
$match: {
$expr: {
$eq: [ "$lesson_id", "$$lesson_id" ]
}
}
},
{
$project: {
title:1,
check_list:1,
duration:1
}
},
{ $sample: { size: '$$limit' } }
],
as: 'quiz'
}
},
{$unwind: '$quiz'},
{ $replaceRoot: { newRoot: "$quiz" } }
]).exec();
The error said size argument to $sample must be a number
Here is my sample data

Answer 1

UPDATE
I think your main problem is to randomly pick amount number of quizs under each lesson. Since $sample is not helpful use $function (New in version MongoDB 4.4).
Solution
Inside $function operator write some logic to
Shuffle the questions (You can change it to your requirement).
Slice it to return the number(amount) of questions required.
db.lessons.aggregate([
{ $match: { "quiz.status": 1 } },
{
$lookup: {
from: "quiz",
let: { "lesson_id": "$_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$lesson_id", "$$lesson_id"] }
}
},
{
$project: {
title: 1,
check_list: 1,
duration: 1
}
}
],
as: "questions"
}
},
{
$project: {
quiz: {
$function: {
body: function(questions, amount) {
if (amount == 0) return [];
for (let i = questions.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[questions[i], questions[j]] = [questions[j], questions[i]];
}
return questions.slice(0, amount);
},
args: ["$questions", { $ifNull: ["$quiz.amount", 0] }],
lang: "js"
}
}
}
},
{ $unwind: "$quiz" },
{ $replaceRoot: { newRoot: "$quiz" } }
]);
$sample does not support variables. A number must be specified explicitly like:
{
$sample: { size: 1 }
}
Also replace your let as shown below because last lesson document has no amount filed in the quiz object
let: {
'lesson_id': '$_id',
'limit': { $ifNull: ['$quiz.amount', 0] } // or any other number.
},
Wrong:
{
$sample: { size: "$$limit" } // Wont work!
}