MongoDB通过查找查询随机选择5个文档

Question

I need to find random 5 documents from mongoDB by using find function. I using LoopBack 4 framework. I already try to use sample (it is in comment)

 const userParties: IndividualParty[] = (await find(
            this.logger,
            {
                where: {
                    and: [
                        { _id: { nin: ids.map(id => id) } },
                        { gender: { inq: gender } },
                    ],
                },
                //sample: { size: 5 },
                //limit: 5,
            } as Filter<IndividualParty>,
            this.partyRepository,
        )) as IndividualParty[];

Answer 1

I'm not familiar with Loopback, but using pure node and node MongoDB driver, here's the shortest example I can come up with:

var run = async function() {
  const conn = await require('mongodb').MongoClient.connect('mongodb://localhost:27017', {useNewUrlParser: true})
  let agg = [
    {'$match': {'_id': {'$gte': 50}}},
    {'$sample': {'size': 5}}
  ]
  let res = await conn.db('test').collection('test').aggregate(agg).toArray()
  console.log(res)
  await conn.close()
}()

In a collection containing _id from 0 to 99, this will randomly output 5 documents having _id larger than 50. Example output:

[ { _id: 60 }, { _id: 77 }, { _id: 84 }, { _id: 96 }, { _id: 63 } ]

You would need to make the above example work with Loopback, but the basic idea is there.

Note:

You need aggregation instead of find() .

Have a read through the $sample documentation and note especially its behavior :

$sample uses one of two methods to obtain N random documents, depending on the size of the collection, the size of N, and $sample's position in the pipeline.

The position of $sample in the pipeline is important. If you need to select a subset of the collection to do $sample on via a $match stage (as with the example above), then you will need to ensure that the subset to be sampled is within 16 MB (the limit of MongoDB in-memory sort).