$center

1.4 新版功能.

This specifies a circle shape for the $within operator in geospatial queries. To define the bounds of a query using $center, you must specify:

• the center point, and
• the radius

Considering the following example:

db.collection.find( { location: { $within: { $center: [ [0,0], 10 ] } } } );

The above command returns all the documents that fall within a 10 unit radius of the point [0,0].

在 2.2.3 版更改: Before 2.2.3, a geospatial index must exist on a field holding coordinates before using any of the geolocation query operators. After 2.2.3, applications may use geolocation query operators without having a geospatial index; however, geospatial indexes will support much faster geospatial queries than the unindexed equivalents.