ruby-on-rails - 如何使嵌入的范围只获取第一个元素？

Question

我有很多坐标嵌入到位。如何只获得每个自放置对象的第一个“开始”坐标？范围是正确的想法吗？我只能选择第一个、最后一个或所有具有所有(非常慢)坐标的位置，我注释掉的范围不起作用。

代码:

class Place
  include Mongoid::Document
  field :title, :type => String
  embeds_many :coordinates
  # def self.start_coordinate
  #   self.coordinates.first
  #   first = self.coordinates.first
  ##  first = self.find({}, { "coordinates" => { "_id" => firstobj?}})
  ##  first = self.find({}, { "coordinates" => {}, :limit=>1})
  ##  self.includes(:coordinates).first
  ##  self.collection.(:coordinates).find_one()
  ### self.all
  # end
end

class Coordinate
  include Mongoid::Document
  include Mongoid::Spacial::Document
  field :coordinates, :type => Array, spacial: true
  spacial_index :coordinates
  embedded_in :place, :inverse_of => :coordinate
end

MongoDB 放置对象:

{ "_id" : ObjectId( "4ece5a04ca6a175b08000016" ),
  "coordinates" : [ 
    { "lat" : 51.54983275438141,
      "lng" : 17.31981750561522,
      "_id" : ObjectId( "4ece5a04ca6a175b08000002" ) }, 
    { "lat" : 51.55282151156834,
      "lng" : 17.35552307202147,
      "_id" : ObjectId( "4ece5a04ca6a175b08000003" ) }, 
    { "lat" : 51.53830285151265,
      "lng" : 17.39397522045897,
      "_id" : ObjectId( "4ece5a04ca6a175b08000004" ) } ],
  "created_at" : Date( 1322146308000 ),
  "description" : "description",
  "title" : "test",
  "updated_at" : Date( 1322154405000 ),
  "user_id" : ObjectId( "4ecd7d4eca6a175783000010" ) }

Answer 1

范围通常用于根据某些条件过滤您想要的整个 对象的结果集。相反，看起来您正在尝试选择给定文档的某些属性。为此，您应该使用 only 方法。

这可能有效，但不确定确切的语法:

Place.only(":coordinates.0")