mongodb - concat array field from many documents to one array field

Question

I have this query:

 [
  {
    '$match': {
      'imageUrls': {
        '$type': 'array', 
        '$ne': []
      }
    }
  }, {
    '$project': {
      'imageUrls': 1, 
      'time': 1
    }
  }, {
    '$sort': {
      'time': 1
    }
  }, {
    '$skip': 0
  }, {
    '$limit': 10
  }
]

This is the schema:

{
    "_id": {
        "$oid": "5ffb0c14dd482daa039ee906"
    },
    "imageUrls": ["image url1", "image url2"],
    "time": 342432432432
}

The output is a list of documents, each containing the array field imageUrls. I want to have all the values in the imageUrls fields in one single array field.

Possible duplicate stackoverflow.com/questions/19266590/…

chridam
– chridam

2021-01-19 15:58:40 +00:00
Commented Jan 19, 2021 at 15:58 — chridam
– chridam, Commented Jan 19, 2021 at 15:58

turivishal · Accepted Answer · 2021-01-19 16:37:07Z

2

$sort by time in descending order
$project to show only one field imageUrls
$unwind deconstruct imageUrls array
$facet to separate both result, total and pagination documents
$skip documents
$limit number of documents
$group by null and reconstruct imageUrls array

  { $sort: { time: -1 } },
  { $project: { imageUrls: 1 } },
  { $unwind: "$imageUrls" },
  {
    $facet: {
      result: [
        { $skip: 0 },
        { $limit: 10 },
        {
          $group: {
            _id: null,
            imageUrls: { $push: "$imageUrls" }
          }
        }
      ],
      count: [
        { $count: "count" }
      ]
    }
  }

Playground

edited Jan 19, 2021 at 16:37

answered Jan 19, 2021 at 16:19

turivishal

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5 Comments

Rony Tesler Over a year ago

I want to sort the results by time, so when exactly should I use these stages inside mine, or maybe I should move the $sort stage to somewhere earlier?

Rony Tesler Over a year ago

so the skip and limit I should put right before the group, right?

Rony Tesler Over a year ago

And how do I get the total count of all urls, ignoring the $skip and $limit?

Haniel Baez Over a year ago

You should add the $limit before the $sort, because the resulting documents must not exceed the BSON Document Size limit of 16 megabytes.

turivishal Over a year ago

@RonyTesler i have updated answer, i hope you have no more questions after this.

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