How to reference an embedded document in Mongoid?

Question

Using Mongoid, let's say I have the following classes:

class Map
  include Mongoid::Document

  embeds_many :locations
end

class Location
  include Mongoid::Document

  field :x_coord, :type => Integer
  field :y_coord, :type => Integer

  embedded_in      :map, :inverse_of => :locations
end


class Player
  include Mongoid::Document

  references_one   :location
end

As you can see, I'm trying to model a simple game world environment where a map embeds locations, and a player references a single location as their current spot.

Using this approach, I'm getting the following error when I try to reference the "location" attribute of the Player class:

Mongoid::Errors::DocumentNotFound: Document not found for class Location with id(s) xxxxxxxxxxxxxxxxxxx.

My understanding is that this is because the Location document is embedded making it difficult to reference outside the scope of its embedding document (the Map). This makes sense, but how do I model a direct reference to an embedded document?

Answer 1

Because Maps are their own collection, you would need to iterate over every Map collection searching within for the Location your Player is referenced.

You can't access embedded documents directly. You have to enter through the collection and work your way down.

To avoid iterating all of the Maps you can store both the Location reference AND the Map reference in your Player document. This allows you to chain criteria that selects your Map and then the Location within it. You have to code a method on your Player class to handle this.

def location
  self.map.locations.find(self.location_id)
end

So, similar to how you answered yourself except you could still store the location_id in your player document instead of using the coord attribs.

Another way would be to put Maps, Locations, and Players in their own collections instead of embedding the Location in your Map collection. Then you could use reference relationships without doing anything fancy... however your really just using a hierarchical database likes it's a relational database at this point...

Answer 2

PLEASE go and VOTE for the "virtual collections" feature on MongoDB's issue tracker:

http://jira.mongodb.org/browse/SERVER-142

It's the 2nd most requested feature, but it still hasn't been scheduled for release. Maybe if enough people vote for it and move it to number one, the MongoDB team will finally implement it.

Answer 3

In my use case, there is no need for the outside object to reference the embedded document. From the mongoid user group, I found the solution: Use referenced_in on the embedded document and NO reference on the outside document.

class Page
  include Mongoid::Document
  field :title

  embeds_many :page_objects
end

class PageObject
  include Mongoid::Document
  field :type

  embedded_in       :page,    :inverse_of => :page_objects
  referenced_in     :sprite
end

class Sprite
  include Mongoid::Document
  field :path, :default => "/images/something.png"
end

header_sprite = Sprite.create(:path => "/images/header.png")
picture_sprte = Sprite.create(:path => "/images/picture.png")

p = Page.create(:title => "Home")
p.page_objects.create(:type => "header", :sprite => header_sprite)
p.page_objects.first.sprite == header_sprite

Answer 4

My current workaround is to do the following:

class Map
  include Mongoid::Document

  embeds_many     :locations
  references_many :players, :inverse_of => :map
end

class Player
  referenced_in :map
  field :x_coord
  field :y_coord

  def location=(loc)
    loc.map.users << self
    self.x_coord = loc.x_coord
    self.y_coord = loc.y_coord
    self.save!
  end

  def location
    self.map.locations.where(:x_coord => self.x_coord).and(:y_coord => self.y_coord).first
  end  
end

This works, but feels like a kluge.

Answer 5

Thinking outside the box, you could make Location its own document and use Mongoid Alize to automatically generate embedded data in your Map document from your Location documents.

https://github.com/dzello/mongoid_alize

The advantage of this method is that you get efficient queries when conditions are suitable, and slower reference based queries on the original document when there's no other way.