How to make scope from embedded_in that get only first element? - ruby-on-rails

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")