$center¶

Definition定义¶

$center¶

The $center operator specifies a circle for a $geoWithin query. $center运算符为$geoWithin查询指定一个圆。The query returns legacy coordinate pairs that are within the bounds of the circle. 该查询返回圆边界内的旧坐标对。The operator does not return GeoJSON objects.运算符不返回GeoJSON对象。

To use the $center operator, specify an array that contains:要使用$center运算符，请指定包含以下内容的数组：

• The grid coordinates of the circle’s center point, and圆中心点的栅格坐标，以及
• The circle’s radius, as measured in the units used by the coordinate system.以坐标系使用的单位测量的圆半径。

{
   <location field>: {
      $geoWithin: { $center: [ [ <x>, <y> ] , <radius> ] }
   }
}

Important

If you use longitude and latitude, specify longitude first.如果使用经度和纬度，请先指定经度。

Behavior行为¶

The query calculates distances using flat (planar) geometry.该查询使用平面几何图形计算距离。

Applications can use $center without having a geospatial index. 应用程序可以使用$center，而无需地理空间索引。However, geospatial indexes support much faster queries than the unindexed equivalents.然而，地理空间索引支持的查询速度要比未索引的索引快得多。

Only the 2d geospatial index supports $center.只有2d地理空间索引支持$center。

Example示例¶

The following example query returns all documents that have coordinates that exist within the circle centered on [ -74, 40.74 ] and with a radius of 10:以下示例查询返回所有文档，这些文档的坐标位于以[ -74, 40.74 ]为中心的圆内，半径为10：

db.places.find(
   { loc: { $geoWithin: { $center: [ [-74, 40.74], 10 ] } } }
)