MongoDB batch get for DataLoader

Question

I actually resolved this issue, however, posting here as a question in hope someone will find the solution useful. I was looking for a purely MongoDB query/aggregation that would serve as a batch getter for FB's DataLoader. I didn't want to .map() afterwards on the server, so the query result had to be an object with keys being the ids, and the values being the documents themselves.

Answer 1

const resMapArr = db.SOMECOLLECTION.aggregate([{
        $match : {
            _id: { $in: _ids }
        }
      }, {
        $group: {
            _id: null, 
            docs: { $push: "$$ROOT" }
        }
      }, {
        $project: {
          res: {
            $arrayToObject: {
              $map: {
                input: "$docs",
                as: "el",
                in: {
                  k: { $convert: { input: "$$el._id", to: "string" }},
                  v: "$$el"
                }
              }
            }
          }
        }
      }, {
        $replaceRoot: { newRoot: "$res" }
      } 
    ])
const resMap = resMapArr[0];

This way you can return the results in DataLoader's batch function as:

return _ids.map(id => resMap[id.toHexString()] || null);