MongoDB 中如何测量 $maxDistance

在MongoDB中，$maxDistance是一个用于测量地理位置之间最大距离的操作符。它通常与$near操作符一起使用，用于查找指定地理位置附近的文档。$maxDistance的值表示了最大距离的范围，可以通过设置不同的值来限制搜索结果的范围。

javascript
db.places.find({
  location: {
    $near: {
      $geometry: {
        type: "Point",
        coordinates: [longitude, latitude]
      },
      $maxDistance: distance
    }
  }
})

javascript
db.cities.insertMany([
  {
    name: "New York",
    location: {
      type: "Point",
      coordinates: [-74.0059, 40.7128]
    }
  },
  {
    name: "Los Angeles",
    location: {
      type: "Point",
      coordinates: [-118.2437, 34.0522]
    }
  },
  {
    name: "Chicago",
    location: {
      type: "Point",
      coordinates: [-87.6298, 41.8781]
    }
  },
  {
    name: "San Francisco",
    location: {
      type: "Point",
      coordinates: [-122.4194, 37.7749]
    }
  }
])

javascript
db.cities.find({
  location: {
    $near: {
      $geometry: {
        type: "Point",
        coordinates: [-74.0059, 40.7128]
      },
      $maxDistance: 50000 // 50 km
    }
  }
})