How to iterate through a set to get field value in MongoDB

Question

Can somebody tell me please if is possible to iterate through a set to create a field value for key in mongodb result. If I have $facet state in pipeline like:

'missing': [{'$group': {'_id': '$foo', 'woo': {'$addToSet': '$wwo'}}},
           {'$project': {'missing_woo': {'$setDifference': [woo_set, '$woo']}}

I would like to get result where code value will be the key like

{'missing_woo': 'missing_woo1'}, {'missing_woo': 'missing_woo2'},... {'missing_woo': 'missing_wooN'}

so that I can iterate through the set generated at $project and to create field values

Answer 1

You can simply use $unwind:

db.collection.aggregate([
  {
    $facet: {
      missing: [
        {$group: {_id: "$foo", woo: {$addToSet: "$wwo"}}},
        {$project: {_id: 0, missing_woo: 
          {$setDifference: [
                [
                  "woo1",
                  "woo2",
                  "wooN",
                  "missing_woo1",
                  "missing_woo2",
                  "missing_wooN"
                ],
                "$woo"
              ]
            }
          }
        },
        {$unwind: "$missing_woo"}
      ]
    }
  }
])

See how it works on the playground example