I have a relatively straight forward query, yet I can’t seem to create the proper index to make it the most efficient read that I can (or can’t seem to instruct mongo to use my index). The query is:

const query = {
  'location.geoJson': {
    $geoWithin: {
      $centerSphere: [
        user.location.geoJson.coordinates,
        defaultRadiusInMiles / earthRadiusInMiles,
      ],
    },
  },
  _id: { $lt: lastId },
};

results = collections.myCollection.find(query).sort({ _id: -1 }).limit(limit);

The index I’ve created in attempt to make this query more efficient is

collections.myCollection.createIndex({ 'location.geoJson': '2dsphere', _id: -1 })

However, when I review the explainStats, I see the following:

 "winningPlan": {
      "stage": "LIMIT",
      ...
      "inputStage": {
        "stage": "FETCH",
        "filter": {
          "location.geoJson": {
            "$geoWithin": {
              "$centerSphere": [
              ...
              ]
            }
          }
        },
        "inputStage": {
          "stage": "IXSCAN",
          "keyPattern": {
            "_id": 1
          },
          "indexName": "_id_",

Which according to the documentation, indicates that mongo is first doing an index scan on _id, THEN fetching based on location, and lastly limiting the results, which is not what I want.

So, is this because my compound index is incorrect and not supporting this query? Or how can I force mongo to use my compound index? Lastly, the queryPlanner shows that the parsed query is:

"parsedQuery": {
      "$and": [
        {
          "_id": {
            "$lt": ...
          }
        },
        {
          "location.geoJson": {
            "$geoWithin": {
              "$centerSphere": [
                ...
              ]
            }
          }
        }
      ]
    },

Maybe because the first element in the $and array is the _id portion of the query, that’s why it’s executing that first? Any help whatsoever is appreciated.