Question

I have many coordinates embedded in place. How to get only first “start” coordinate for each self Place object? Scope is correct idea? I can select only first, last or all of the places with all(is very slow) coordinates, my commented out scope doesn’t work.

code:

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 place object:

{ "_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

Scopes are typically used to filter the result set of whole objects you want based upon some criteria. Instead, it looks like you are trying to select certain attributes of a given document. To do that you should use the only method.

This might work, not sure about the exact syntax though:

Place.only(":coordinates.0")