Aggregation from two collections?

Question

MongoDB

Guys, help to form a request. There are two collections with the following document structures

db.xcover
{
    "_id" : ObjectId("5b101caddcab7850a4ba32eb"),
    "connections" : [
        169906699,
        125629607
    ],
    ...other-fields
}

db.dcover
{
    "_id" : 169906699,
    "changed" : true,
    "time" : false,
    ...other-fields
}

connection db.dcover._id in db.xcover.connections

And in essence I need the following, further query pseudocode:

db.dcover.aggregate([
  {
    $match: {
        $or: [{ changed: true }, { time: true }]
    }
  },
  {
    $lookup: {
      from: 'xcover',
      localField: '_id',
      pipeline: [
        { $match: { $expr: { $in: ['$_id',  '$xcover.connections'] } } },
        { $unwind: '$xcover' }
      ],
       as: 'xcover'
     }
  }
])

Those. i want to fetch from dcover where changed or time field is set to true and aggregate with data from xcover in array of connections field which has corresponding dcover document IDs.

Answer 1

here is the solution:

db.dcover.aggregate([
     { $match: { $or: [{ changed: true },{ time: true }] } },
     {
         $lookup: {
              from: 'xcover',
              let: { group_id: '$_id' },
              pipeline: [
                 { $match: { $expr: { $in: [ '$$group_id', '$connections' ] }}}
              ],
              as: 'xcovers'
          }
     }
])