class Company
  include Mongoid::Document

  field :name, type: String

  embeds_many :operations
end

class Operation
  include Mongoid::Document

  field :name, type: String

  has_many :inventory_operations

  embedded_in :company
end

class InventoryOperation
  include Mongoid::Document
  field :quantity, type: Integer, default: 0

  belongs_to :inventory
  belongs_to :operation
end

[1] pry(main)> io = InventoryOperation.all[0]
=> #<InventoryOperation _id: 51c859a6280b745f87000002, quantity: 400,inventory_id: "51d09bf6280b745f5b000001", operation_id: "51a721f0236f4483a2000002">

**[2] pry(main)> Operation.find(io.operation_id)**
Mongoid::Errors::DocumentNotFound: 
Problem:
  Document(s) not found for class Operation with id(s) 51a721f0236f4483a2000002.
Summary:
  When calling Operation.find with an id or array of ids, each parameter must match a document in the database or this error will be raised. ....
Resolution:
  Search for an ...
from /Users/xxx/.rvm/gems/ruby-1.9.3-head/gems/mongoid-3.1.4/lib/mongoid/criteria.rb:464:in `check_for_missing_documents!'

[3] pry(main)> os = Company.first.operations.map { |e| e.id};

**[4] pry(main)> Operation.find(io.operation_id)**
=> #<Operation _id: 51a721f0236f4483a2000002, created_at: 2013-05-30 09:54:56 UTC, updated_at: 2013-09-06 16:24:48 UTC, name: "Admin">
[5] pry(main)>