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ActiveRecord plugin to provide a smooth migration from DataMapper to ActiveRecord


~> 0.9
~> 2.0


< 5.0, >= 3.2
~> 3.0
~> 1.5
~> 1.3
~> 2.1
 Project Readme


ActiveRecord plugin to provide a smooth migration from DataMapper to ActiveRecord.

This code is currently in development and not feature complete. Your code is probably different than mine, so please contribute to bridge the gap from DataMapper to ActiveRecord and Rails 4.


Ardm is intended for applications running Rails with DataMapper 1.2 who need to migrate to ActiveRecord so they can move forward onto Rails 4. Lets examine some of the reasons why you might move to ActiveRecord.

  • DataMapper is no longer under development. Ruby Object Mapper (ROM) is the implicit replacement for DataMapper, but it's not a supported migration. ROM is a completely new codebase and very few of the idioms transfer.
  • DataMapper produces inefficient queries. Includes, joins, and subqueries are either not supported or incorrect. Enabling subqueries in DM speeds up queries but causes other subtle query problems than can produce bad SQL that may select incorrect records.
  • DataMapper cannot currently run on Rails 4. Someone may take the initiative to upgrade DataMapper to work on Rails4, but I think you're much better off moving to ActiveRecord.
  • Arel is awesome. ActiveRecord and Arel together is quite nice. With the added support properties and the advances in Rails 4, I think this upgrade is a must.
  • ActiveRecord is used in more applications, is better tested, and is more performant than DataMapper.


Near the top of your config/application.rb, add the following:

require 'ardm'
Ardm.orm = ENV['ORM'] || :dm # default do # only executed if ORM is ActiveRecord
  Bundler.require(:active_record) # libs related to active record
  require "active_record/railtie"
end do # only executed if ORM is DataMapper
  Bundler.require(:data_mapper) # libs related to data mapper
Ardm.setup # this requires the Ardm shim code

Next you need to change add a base class to EVERY model that previously included DataMapper::Resource.

This is very tedious but there's no way around it. All ActiveRecord models must inherit from ActiveRecord::Base, so Ardm creates an interchangeable base class that can be either ActiveRecord or DataMapper.

If your model is STI, add it to the base model.

I'm sorry, it's the only way.


class MyModel < Ardm::Record # <--- add this
  include DataMapper::Resource # you can remove this (it should also be shimmed)

Run your project using the ORM environment variable.

# Try to get this one working now with the changes above.
ORM=dm bundle exec rake spec

# This will fail horribly
ORM=ar bundle exec rake spec

With this new base clase and the ORM environment variable, you can switch between ActiveRecord and DataMapper by flipping the ORM variable.

This approach allows you to continue developing your application in DataMapper while you work on removing all the "datamapper-isms" from your code. This library attempts to take care of most DataMapper features, but there are probably tons of small variations that are not accounted for.

General Strategy

  1. Get the application running in DataMapper with Ardm installed. Don't even think about ActiveRecord until you have Ardm working in DataMapper mode and you can deploy your application normally with ardm installed.
  2. Start to remove references to DataMapper by using the conversions mentioned below. The idea is to remove the DataMapper constant completely so you can run without dm-core when in ActiveRecord mode.
  3. Once your application runs smoothly in DataMapper mode with all the constants using Ardm, try to get the application to start the test run in ActiveRecord mode. This will probably require a bunch of hunting through the application for datamapper-isms that are not accounted for in Ardm. Please help by contributing these fixes back to Ardm!
  4. Make all your tests pass in ActiveRecord and DataMapper mode. This is an ideal. You could decide that you're close enough and start sacrificing DataMapper specific code for ActiveRecord code. You can branch around picky code with the Ardme.activerecord? and Ardm.datamapper? helpers.

This is a complex thing to approach. I hope Ardm will make this change into a repeatable strategy rather than everyone needing to create their own unique solution.


Things that access DataMapper directly can be replaced with Ardm invocations:

DataMapper.finalize    =>  Ardm::Record.finalize # this is still important for Ardm
DataMapper.repository  =>  Ardm::Record.repository
DataMapper.logger      =>  Ardm::Record.logger

DataMapper::ObjectNotFoundError => Ardm::Record::NotFound
ActiveRecord::RecordNotFound    => Ardm::Record::NotFound

DataMapper::Property           =>  Ardm::Property
DataMapper::Property::String   =>  Ardm::Property::String

This pattern follows for most DataMapper module methods. When running in DataMapper mode, these simply forward. In ActiveRecord mode, they provide adapters for accessing the same data through ActiveRecord.

If you run into code that is particularly difficult to convert, you can duplicate the code and write a different version for each ORM: { Thing.where(Thing.arel_table[:field].matches('something')) }
# This is just an example. This should actually work fine in Ardm. { Thing.all( => 'something') }


This is an adaptation of the original DataMapper source code to allow users of the now defunct DataMapper to migrate to ActiveRecord. Much of this code was originally written by Sam Smoot and Dan Kubb as dm-types and dm-core. See LICENSE for details.