MongoDB agregation with filtering in array

Question

I need to count records grouped by tags and have filtered bofore including in ones

// in db
{tags: ['video', 'Alex'], ... },
{tags: ['video', 'John'], ... },
{tags: ['video', 'John'], ... },
{tags: ['text', 'Alex'], ... },
{tags: ['text', 'John'], ... },


client.db('mydb').collection('Files').aggregate(
        [
          { $group: { _id: { tags: '$tags' }, total: { $sum: 1 } } },
          { $match: { tags: 'video' } },
        ],
      ).toArray()

But sadly I got zero docs. If remove $group section I got 3 docs. In original request I anticipated 2 docs

{ _id: ['video', 'Alex'], total: 1 },
{ _id: ['video', 'John'], total: 2 }

ambianBeing · Accepted Answer · 2020-10-09 12:56:48Z

1

In aggregation the order of pipeline is important, as output of previous stage is fed to the next one.

Your query is almost there basis the expected output. Just move $match stage before the $group stage.

Query:

db.collection.aggregate([
  {
    $match: {
      "tags": "video"
    }
  },
  {
    $group: {
      _id: {
        tags: "$tags"
      },
      total: {
        $sum: 1
      }
    }
  }
]);

Working Example

answered Oct 9, 2020 at 12:56

ambianBeing

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