I'm using https://mongoosejs.com/ for querying mongo
I want to find data of array elements.Like this:
var a = ["a","b","c"];
topic.find({topic:a}).limit(4).exec(.....
If I use like this, I can find only for a element; but I need altogether. That means:
limit for a=4 limit for b=4 limit c=4
At stackoverflow you ask a question and we answer the question. Maybe one answer may be 2 may be 3 and stackoverflow send all comments of answers with limit.And i want to do this.
You could use $facet for this which will give you a single result document:
db.collection.aggregate({
$facet: {
"a": [{ $match: { "topic": "a" } }, { $limit: 4 }],
"b": [{ $match: { "topic": "b" } }, { $limit: 4 }],
"c": [{ $match: { "topic": "c" } }, { $limit: 4 }],
}
})
If you need separate documents you would probably append the following stages at the end of the above pipeline:
{
$project: {
"result": { $concatArrays: [ "$a", "$b", "$c" ] }
}
}, {
$unwind: "$result"
}
$facet exists since v3.4.I think the solution here would be to concatenate the results.
const aIds = topic.find({topic:"a"}).limit(4).map( el => el._id );
const bIds = topic.find({topic:"b"}).limit(4).map( el => el._id );
const cIds = topic.find({topic:"c"}).limit(4).map( el => el._id );
topic.find({_id: {$in: aIds.concat(bIds).concat(cIds) }}).exec(.....
Not the most efficient, but it will work.
UPDATE
Initially, I was not aware that this question related to the mongoose. I'm not familiar with it but the idea remains the same: find 3 topics (a,b,c) each with its own limit then join them.
