mongodb中使用limit时如何获取匹配文档的真实数量？

Question

I have a facet aggregation in mongodb. It works. Now, I need to limit the result in order to have an infinite scroll. How to get the total count of matching documents in the entire collection?

I can get the total count of documents in the collection, but this number is not relevant. My goal is to know if the user can fetch more data. If he has received 120 documents, but 150 matches his filter, then another query can be performed.

Here is my query:

 db.collection.aggregate([ { $facet: { data: [ { $match: { score: {$gt: 80 } } }, { $skip: 0}, { $limit: 2}, { $project: { score: 1 } } ], } }, ])

You can play with fake data in this sandbox: https://mongoplayground.net/p/EClbe0f_Fms

The real number of matching documents is 4. If I add a "hits" after the facet with the $count operator, I receive 7, which the the total of documents. How to get this information in an efficient way?

Answer 1

The most efficient way would be not to use $count at all but rather to check if the last page received was less than the limit. Ie:

const perPage = 10;
const results = await getPageOfResults();
const atEnd = results.length < perPage;

It sound like you added $count to your facet already but got an incorrect result. To get the 'real' number of matching documents you need to add the $match stage to both facets like so https://mongoplayground.net/p/liQaiyZhYER

db.collection.aggregate([
  {
    $facet: {
      data: [
        { $match: { score: {$gt: 80 } } },
        { $skip: 0},
        { $limit: 2},
        { $project: { score: 1 } }
      ],
      count: [
        { $match: { score: {$gt: 80 } } },
        { $count: 'total' } 
      ]
    }
  },
  {$unwind: "$count"},
  {$addFields: {
     count: "$count.total"
  }}
])