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Learn more about Teams db.test.find({$or:[{category_id:2},{category_id:2}]).sort({createAt:-1}).limit(5); or db.test.find({category_id:{$in:[1,2]}}).sort({createAt:-1}).limit(5); – matthPen Mar 28, 2019 at 16:47 @matthPen In this way, it is possible that I get 7 entries from category 1 and 3 entries from category 2. What I want is same number entries from each category. – Elect2 Mar 29, 2019 at 0:49 { "$project": { "data": { "$concatArrays": ["$first", "$second"] }}}, { "$unwind": "$data" }, { "$replaceRoot": { "newRoot": "$data" }}

Update

Using simple javascript

const test1 = await db.test.find({ category_id: 1 }).sort({ createAt: -1 }).limit(5)
const test2 = await db.test.find({ category_id: 1 }).sort({ createAt: -1 }).limit(5)
const test = test1.concat(test2)
console.log(test)
                I can get the right result using this query. But it doesn't use my Index({category_id:1,createAt:-1}), and run very slow(around 2 second on a 1million docs collection). If I use general find().sort().limit(), it will use index and run fast(0.001 second)
– Elect2
                Mar 29, 2019 at 1:51
                $facet aggregation does not use the index. You have to manually concat the arrays using javascript. I have updated my answer
– Ashh
                Mar 29, 2019 at 7:41
        
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