$geoNear (aggregation stage)（聚合阶段）

Maximum Distance最大距离

The places collection above has a 2dsphere index. 上面的places集合有一个2dsphere索引。The following aggregation uses $geoNear to find documents with a location at most 2 meters from the center [ -73.99279 , 40.719296 ] and category equal to Parks.以下聚合使用$geoNear查找位置距离中心最多2米的文档[ -73.99279 , 40.719296 ]，category等于Parks（公园）。

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

The aggregation returns the following:聚合返回以下内容：

{
   "_id" : 8,
   "name" : "Sara D. Roosevelt Park",
   "category" : "Parks",
   "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:匹配的文档包含两个新字段：

• dist.calculated field that contains the calculated distance, and包含计算距离的字段，以及
• dist.location field that contains the location used in the calculation.包含计算中使用的位置的字段。

Minimum Distance最小距离

The following example uses the option minDistance to specify the minimum distance from the center point that the documents can be. 以下示例使用选项minDistance指定文档与中心点的最小距离。The following aggregation finds all documents with a location at least 2 meters from the center [ -73.99279 , 40.719296 ] and category equal to Parks.以下聚合查找距离中心[-73.99279, 40.719296]至少2米，category等于Parks的所有文件。

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

$geoNear with the let option$geoNear带let选项

In this example:在这个例子中：

• The let option is used to set an array value of [-73.99279,40.719296] to the variable $pt.let选项用于为变量$pt设置数组值[-73.99279,40.719296]。
• $pt is specified as a let option to the near parameter in the $geoNear stage.$pt被指定为$geoNear阶段near参数的let选项。

db.places.aggregate(
[
   {
      "$geoNear":
      {
         "near":"$$pt",
         "distanceField":"distance",
         "maxDistance":2,
         "query":{"category":"Parks"},
         "includeLocs":"dist.location",
         "spherical":true
      }
   }
],
{
   "let":{ "pt": [ -73.99279, 40.719296 ] }
}
)

The aggregation returns all documents with:聚合返回所有具有以下内容的文档：

• A location at most 2 meters from the point defined in the let variable距离let变量中定义的点最多2米的位置
• A category equal to Parks.一个与Parks相等的category。

{
   _id: ObjectId("61715cf9b0c1d171bb498fd7"),
   name: 'Sara D. Roosevelt Park',
   location: { type: 'Point', coordinates: [ -73.9928, 40.7193 ] },
   category: 'Parks',
   distance: 1.4957325341976439e-7,
   dist: { location: { type: 'Point', coordinates: [ -73.9928, 40.7193 ] } }
},
{
   _id: ObjectId("61715cf9b0c1d171bb498fd6"),
   name: 'Central Park',
   location: { type: 'Point', coordinates: [ -73.97, 40.77 ] },
   category: 'Parks',
   distance: 0.0009348548688841822,
   dist: { location: { type: 'Point', coordinates: [ -73.97, 40.77 ] } }
}

$geoNear with Bound let Option$geoNear，带绑定let选项

The let option can bind a variable which can be used in a $geoNear query.let选项可以绑定一个可用于$geoNear查询的变量。

In this example, $lookup uses:在此示例中，$lookup使用：

• let to define $pt.用来定义$pt。
• $geoNear in the pipeline.在pipeline中。
• $pt to define near in the $geoNear pipeline stage.$pt用于在$geoNear管道阶段定义near。

db.places.aggregate( [
   {
      $lookup: {
         from: "places",
         let: { pt: "$location" },
         pipeline: [
            {
               $geoNear: {
                  near: "$$pt",
                  distanceField: "distance"
               }
            }
         ],
         as: "joinedField"
      }
   },
   {
      $match: { name: "Sara D. Roosevelt Park" }
   }
] );

The aggregation returns a document with:聚合返回一个文档，其中包含：

• The 'Sara D. Roosevelt Park' document as the main document.“萨拉·D·罗斯福公园”文档为主要文档。
• Every document in the places collection as subDocuments using the $pt variable for calculating distance.places集合中的每个文档都作为子文档使用$pt变量计算距离。

{
   _id: ObjectId("61715cf9b0c1d171bb498fd7"),
      name: 'Sara D. Roosevelt Park',
      location: { type: 'Point', coordinates: [ -73.9928, 40.7193 ] },
      category: 'Parks',
      joinedField: [
         {
         _id: ObjectId("61715cf9b0c1d171bb498fd7"),
         name: 'Sara D. Roosevelt Park',
         location: { type: 'Point', coordinates: [ -73.9928, 40.7193 ] },
         category: 'Parks',
         distance: 0
         },
         {
         _id: ObjectId("61715cf9b0c1d171bb498fd6"),
         name: 'Central Park',
         location: { type: 'Point', coordinates: [ -73.97, 40.77 ] },
         category: 'Parks',
         distance: 5962.448255234964
         },
         {
         _id: ObjectId("61715cfab0c1d171bb498fd8"),
         name: 'Polo Grounds',
         location: { type: 'Point', coordinates: [ -73.9375, 40.8303 ] },
         category: 'Stadiums',
         distance: 13206.535424939102
         }
      ]
}

Specify Which Geospatial Index to Use指定要使用的地理空间索引

Consider a places collection that has a 2dsphere index on the location field and a 2d index on the legacy field.考虑一个places集合，它在位置字段上有一个2dsphere索引，在legacy字段上有个2d索引。

A document in the places collection resembles the following:places集合中的文档类似于以下内容：

{
   "_id" : 3,
   "name" : "Polo Grounds",
   "location": {
      "type" : "Point",
      "coordinates" : [ -73.9375, 40.8303 ]
   },
   "legacy" : [ -73.9375, 40.8303 ],
   "category" : "Stadiums"
}

The following example uses the key option to specify that the aggregation should use the location field values for the $geoNear operation rather than the legacy field values. 以下示例使用key选项指定聚合应使用$geoNear操作的location字段值，而不是legacy字段值。The pipeline also uses $limit to return at most 5 documents.该管道还使用$limit最多返回5个文档。

db.places.aggregate([
   {
     $geoNear: {
        near: { type: "Point", coordinates: [ -73.98142 , 40.71782 ] },
        key: "location",
        distanceField: "dist.calculated",
        query: { "category": "Parks" }
     }
   },
   { $limit: 5 }
])

The aggregation returns the following:聚合返回以下内容：

{
   "_id" : 8,
   "name" : "Sara D. Roosevelt Park",
   "location" : {
      "type" : "Point",
      "coordinates" : [
         -73.9928,
         40.7193
      ]
   },
   "category" : "Parks",
   "dist" : {
      "calculated" : 974.175764916902
   }
}
{
   "_id" : 1,
   "name" : "Central Park",
   "location" : {
      "type" : "Point",
      "coordinates" : [
         -73.97,
         40.77
      ]
   },
   "legacy" : [
      -73.97,
      40.77
   ],
   "category" : "Parks",
   "dist" : {
      "calculated" : 5887.92792958097
   }
}

Maximum Distance最大距离

Minimum Distance最小距离

The following example returns the first 4 matching documents outside of an 8000 meter radius of the specified point, in order of ascending distance. 以下示例按距离升序返回指定点8000米半径外的前4个匹配文档。The code includes a Query parameter that only matches documents in which the value of the location.address.state field is "NJ". 该代码包含一个Query参数，该参数仅匹配location.address.state字段值为"NJ"的文档。The code also stores the calculated distance in the distance field of the output documents.该代码还将计算出的距离存储在输出文档的distance字段中。

var pipeline = new EmptyPipelineDefinition<Theater>()
    .GeoNear(
        GeoJson.Point(GeoJson.Geographic(-74.1, 40.95)),
        new GeoNearOptions<Theater, Theater>
        {
          DistanceField = "distance",
          MinDistance = 8000,
          Key = "location.geo",
          Query = Builders<Theater>.Filter.Eq(t => t.Location.Address.State, "NJ"),
        })
    .Limit(4);

Maximum Distance最大距离

Minimum Distance最小距离

The following example returns the first 4 matching documents outside of an 8000 meter radius of the specified point, in order of ascending distance. 以下示例按距离升序返回指定点8000米半径外的前4个匹配文档。The code includes a query field that only matches documents where the value of the location.address.state field is "NJ". 该代码包含一个query字段，该字段仅匹配location.address.state字段值为"NJ"的文档。The code also stores the calculated distance in the distance field of the output documents:该代码还将计算出的距离存储在输出文档的distance字段中：

const pipeline = [
  {
    $geoNear: {
      near: {
        type: "Point",
        coordinates: [-74.1, 40.95]
      },
      distanceField: "distance",
      minDistance: 8000,
      query: { "location.address.state": "NJ" },
      spherical: true
    },
  },
  { $limit: 4 }
];

const cursor = collection.aggregate(pipeline);
return cursor;