How to get Dragonfly working on with a Mongo Datastore using Mongoid 3 on both localhost and Heroku. It's a little bit of a pain and here is the quick and dirty approach without Monkey Patching.

dragonfly.rb

require 'dragonfly'
 
app = Dragonfly[:images]
 
mconfig = Mongoid.load!("config/mongoid.yml", Rails.env)
 
app.datastore = Dragonfly::DataStorage::MongoDataStore.new
app.datastore.configure do |c|
 
  cfg = mconfig['sessions']['default']
  if ENV['MONGOLAB_URI']
    # Painful support of Heroku
    uri = cfg['uri']    # => "mongodb://appname:pass@ds034534.mongolab.com:29217/database_name"
    uri = uri.gsub('mongodb://', '')
    user_pass, host_string = uri.split('@')
    username, password = user_pass.split(':')
    host_port, database = host_string.split('/')
    host, port = host_port.split(':')
 
    c.host = host
    c.port = port
    c.database = database
    c.username = username
    c.password = password
  else
    if cfg['hosts'].size < 2
      c.host = cfg['hosts'].first.split(':').first
      c.port = cfg['hosts'].first.split(':').second
    else
      c.hosts = cfg['hosts']
    end
    c.database = cfg['database']
    c.username = cfg['username']
    c.password = cfg['password']
  end
end
app.configure_with(:imagemagick)
app.configure_with(:rails) 
 
# app.configure do |c|
#   c.url_format = '/:job'
# end
 
# Allow all mongoid models to use the macro 'image_accessor'
app.define_macro_on_include(Mongoid::Document, :image_accessor)

mongoid.yml

<% if ENV['MONGOLAB_URI'] %>
production:
  sessions:
    default:
      uri: <%= ENV['MONGOLAB_URI'] %>
      options:
        skip_version_check: true
        safe: true
<% else %>
production:
  sessions:
    default:
      database: my_production
      hosts:
        - localhost:27017
      options:
        skip_version_check: true
        safe: true
<% end %>
<% if ENV['MONGOLAB_URI'] %>
development:
  sessions:
    default:
      uri: mongodb://appname:pass@ds034534.mongolab.com:29217/database_name
      options:
        skip_version_check: true
        safe: true
<% else %>
development:
  # Configure available database sessions. (required)
  sessions:
    # Defines the default session. (required)
    default:
      # Defines the name of the default database that Mongoid can connect to.
      # (required).
      database: my_development
      # Provides the hosts the default session can connect to. Must be an array
      # of host:port pairs. (required)
      hosts:
        - localhost:27017
      options:
        # Change whether the session persists in safe mode by default.
        # (default: false)
        # safe: false
 
        # Change the default consistency model to :eventual or :strong.
        # :eventual will send reads to secondaries, :strong sends everything
        # to master. (default: :eventual)
        # consistency: :eventual
 
        # How many times Moped should attempt to retry an operation after
        # failure. (default: 30)
        # max_retries: 30
 
        # The time in seconds that Moped should wait before retrying an
        # operation on failure. (default: 1)
        # retry_interval: 1
  # Configure Mongoid specific options. (optional)
  options:
    # Configuration for whether or not to allow access to fields that do
    # not have a field definition on the model. (default: true)
    # allow_dynamic_fields: true
 
    # Enable the identity map, needed for eager loading. (default: false)
    # identity_map_enabled: false
 
    # Includes the root model name in json serialization. (default: false)
    # include_root_in_json: false
 
    # Include the _type field in serializaion. (default: false)
    # include_type_for_serialization: false
 
    # Preload all models in development, needed when models use
    # inheritance. (default: false)
    # preload_models: false
 
    # Protect id and type from mass assignment. (default: true)
    # protect_sensitive_fields: true
 
    # Raise an error when performing a #find and the document is not found.
    # (default: true)
    # raise_not_found_error: true
 
    # Raise an error when defining a scope with the same name as an
    # existing method. (default: false)
    # scope_overwrite_exception: false
 
    # Skip the database version check, used when connecting to a db without
    # admin access. (default: false)
    # skip_version_check: false
 
    # User Active Support's time zone in conversions. (default: true)
    # use_activesupport_time_zone: true
 
    # Ensure all times are UTC in the app side. (default: false)
    # use_utc: false
<% end %>
test:
  sessions:
    default:
      database: my_test
      hosts:
        - localhost:27017
      options:
        consistency: :strong
        # In the test environment we lower the retries and retry interval to
        # low amounts for fast failures.
        max_retries: 1
        retry_interval: 0

I have linked my dragonfly/mongo_mapper config for heroku to your gist.

Here is my gist: https://gist.github.com/JonKernPA/9520920