Create MongoDB fields with names based on sub-document values using aggregation pipeline?

Question

Given a MongoDB collection with the following document structure:

{
    array_of_subdocs:
         [
             {
                 animal: "cat",
                 count: 10
             },
             {
                 animal: "dog",
                 count: 20
             },

             ...
         ]
}

where each document contains an array of sub-documents, I want transform the collection into documents of the structure:

{
    cat: { count: 10 },
    dog: { count: 20 },
    ...
}

where each sub-document is now the value of a new field in the main document named after one of the values within the sub-document (in the example, the values of the animal field is used to create the name of the new fields, i.e. cat and dog).

I know how to do this with eval with a Javascript snippet. It's slow. My question is: how can this be done using the aggregation pipeline?

Answer 1

According to this feature request and its solution, the new feature will be added for this functionality - Function called arrayToObject.

db.foo.aggregate([
  {$project: {
    array_of_subdocs: {
      $arrayToObject: {
        $map: {
          input: "$array_of_subdocs",
          as: "pair",
          in: ["$$pair.animal", "$$pair.count"]
        }
      }
    }
  }}
])

But at the moment, no solution. I suggest you to change your data structure. As far as I see there are many feature requests labeled as Major but not done for years.