Using aggregation to create the geospatial field

Using aggregation we can consolidate the geospatial field creation into a single command. It is important to note that the current values for latitude and longitude are stored as a data type string, and hence are unusable for the purposes of geospatial queries. Accordingly, we employ the $toDouble pipeline expression operator. 

Here is the command that adds a new field, geo_spatial_flat, consisting of legacy coordinate pairs, from existing latitude and longitude coordinates in the world_cities collection:

db.world_cities.aggregate(
    { "$match": {} },
    { "$addFields": {
          "geo_spatial_flat" : [
              { "$toDouble" : "$longitude" },
              { "$toDouble" : "$latitude" } ]
          }
    },
    { "$out" : "world_cities" }
);

This command uses three stages: $match, $addFields, and $out:

• The $match stage has an empty filter...