Mongodb dynamic project in aggregate

Question

I have my collection of structure

{ 
  a: 1,
  b: 2,
  c: 3,
  d: 4,
  e: 5,
  allowed: ['a', 'b']
}

In this collection, allowed array stores fields name to be fetched. It is set by the user preference which he wants to fetch and can change those selected fields and is updated in allowed array.

I want to fetch using MongoDB aggregate and project in some way that allowed array will be used without manually listing all the fields in $project

{ $project: { a: 1, b: 1 } }

I have one solution like using below project in aggregate

  db.getCollection("dummy").aggregate([
      { $match: {} },
      {
        $project: {
          a: {
            $cond: {
              if: { $in: ["a", "$allowed"] },
              then: "$a",
              else: "$$REMOVE",
            },
          },
          b: {
            $cond: {
              if: { $in: ["b", "$allowed"] },
              then: "$b",
              else: "$$REMOVE",
            },
          },
        },
      },
    ]);

but this also requires listing all fields. I am looking for an alternate solution that projects fields in the allowed array without listing all fields manually in $project.

Ashh · Accepted Answer · 2020-04-30 16:57:01Z

3

Since you do not know what are the fields in your documents, You have to convert them to k-v pair using $objectToArray then use $filter to filter out the fields which are not in $allowed array. FInally use $arrayToObject to get initial shape.

db.collection.aggregate([
  { $replaceRoot: {
    newRoot: {
      $arrayToObject: {
        $filter: {
          input: { $objectToArray: "$$ROOT" },
          cond: { $in: ["$$this.k", "$allowed"] }
        }
      }
    }
  }}
])

MongoPlayground

edited Apr 30, 2020 at 16:57

answered Apr 30, 2020 at 16:50

Ashh

46.6k16 gold badges111 silver badges137 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ayush Nawani Over a year ago

Thanks a lot for the quick response. It's working. I have learned something new today.

Collectives™ on Stack Overflow

Mongodb dynamic project in aggregate

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related