# app/models/thing.rb
class Thing
  include Mongoid::Document
end

2.5.3 :001 > thing1 = Thing.create!
 => #<Thing _id: 5c45ed89a8f843354e3bc05a, > 
2.5.3 :002 > thing2 = Thing.create!
 => #<Thing _id: 5c45ed8ba8f843354e3bc05b, > 
2.5.3 :003 > Thing.where(id: thing1.id.to_s).count
 => 1 
2.5.3 :004 > Thing.where(id: thing1.id.to_s).map(&:id)
 => [BSON::ObjectId('5c45ed89a8f843354e3bc05a')] 
2.5.3 :005 > Thing.where(id: thing1.id.to_s).find(thing2.id.to_s)         # I'd expect 'nil'
 => #<Thing _id: 5c45ed8ba8f843354e3bc05b, >