mongodb apply sort to lookup results

Question

If I have a user and post collection

{\"_id\": 1, \"name\": \"User 1\"}
{\"_id\": 2, \"name\": \"User 2\"}

{\"_id\": 1, \"title\":         


        

Answer 1

You can solve this in a single aggregation step with the new $lookup syntax

db.getCollection('users').aggregate([{
    '$lookup': {
      'from': 'posts',
      'let': {
        'userId': '$_id'
      },
      'pipeline': [{
          '$match': { '$expr': { '$eq': ['$userId', '$$userId'] } }
        }, {
          '$sort': {  'createdAt': -1 }
        }, {
          '$limit': 10
        },
      ],
      'as': 'posts'
    }
  }
])

Note: untested code, but the principle should be clear.