$geoNear (aggregation)¶

Definition¶

$geoNear¶

2.4 新版功能.

Outputs documents in order of nearest to farthest from a specified point.

The $geoNear stage has the following prototype form:

{ $geoNear: { <geoNear options> } }

The $geoNear operator accepts a document that contains the following $geoNear options. Specify all distances in the same units as those of the processed documents’ coordinate system:

Field	Type	Description
`near`	GeoJSON point or legacy coordinate pairs	The point for which to find the closest documents.
`distanceField`	string	The output field that contains the calculated distance. To specify a field within a subdocument, use dot notation.
`limit`	number	Optional. The maximum number of documents to return. The default value is `100`. See also the `num` option.
`num`	number	Optional. The `num` option provides the same function as the `limit` option. Both define the maximum number of documents to return. If both options are included, the `num` value overrides the `limit` value.
`maxDistance`	number	Optional. The maximum distance from the center point that the documents can be. MongoDB limits the results to those documents that fall within the specified distance from the center point. Specify the distance in meters for GeoJSON data and in radians for legacy coordinate pairs.
`query`	document	Optional. Limits the results to the documents that match the query. The query syntax is the usual MongoDB read operation query syntax. You cannot specify a `$near` predicate in the `query` field of the `$geoNear` stage.
`spherical`	Boolean	Required if using a 2dsphere index. For use with `2dsphere` indexes, `spherical` must be `true`. The default value is `false`.
`distanceMultiplier`	number	Optional. The factor to multiply all distances returned by the query. For example, use the `distanceMultiplier` to convert radians, as returned by a spherical query, to kilometers by multiplying by the radius of the Earth.
`includeLocs`	string	Optional. This specifies the output field that identifies the location used to calculate the distance. This option is useful when a location field contains multiple locations. To specify a field within a subdocument, use dot notation.
`uniqueDocs`	Boolean	Optional. If this value is `true`, the query returns a matching document once, even if more than one of the document’s location fields match the query. 2.6 版后已移除: Geospatial queries no longer return duplicate results. The `$uniqueDocs` operator has no impact on results.

Behavior¶

When using $geoNear, consider that:

You can only use $geoNear as the first stage of a pipeline.
You must include the distanceField option. The distanceField option specifies the field that will contain the calculated distance.
The collection must have a geospatial index.
The $geoNear requires that a collection have at most only one 2d index and/or only one 2dsphere index.
If using a 2dsphere index, you must specify spherical: true.

You cannot specify a $near predicate in the query field of the $geoNear stage.

Generally, the options for $geoNear are similar to the geoNear command with the following exceptions:

distanceField is a mandatory field for the $geoNear pipeline operator; the option does not exist in the geoNear command.
includeLocs accepts a string in the $geoNear pipeline operator and a boolean in the geoNear command.

Example¶

Consider a collection places that has a 2dsphere index. The following aggregation finds at most 5 unique documents with a location at most 2 units from the center [ -73.99279 , 40.719296 ] and have type equal to public:

db.places.aggregate([
   {
     $geoNear: {
        near: { type: "Point", coordinates: [ -73.99279 , 40.719296 ] },
        distanceField: "dist.calculated",
        maxDistance: 2,
        query: { type: "public" },
        includeLocs: "dist.location",
        num: 5,
        spherical: true
     }
   }
])

The aggregation returns the following:

{
   "_id" : 8,
   "name" : "Sara D. Roosevelt Park",
   "type" : "public",
   "location" : {
      "type" : "Point",
      "coordinates" : [ -73.9928, 40.7193 ]
   },
   "dist" : {
      "calculated" : 0.9539931676365992,
      "location" : {
         "type" : "Point",
         "coordinates" : [ -73.9928, 40.7193 ]
      }
   }
}

The matching document contains two new fields:

dist.calculated field that contains the calculated distance, and
dist.location field that contains the location used in the calculation.

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