Using Datamapper with Rails 4

Was trying to swap out active record for datamapper on a rails 4 application this morning and had one or two issues with gem version conflicts (I think this is largely due to DM being discontinued in favour of the not-done-yet ROM). At time of writing if you install with rails 4.0.0 and just go for the rubygems repo you’re going to get moaned at for having incompatible gems.

For a fresh rails 4 application you can get around this by installing from github and specifying “release-1.2” as the branch. I managed to get the full suite up and running by adding the following to my Gemfile (adapted from this guide).

%w{core constraints migrations transactions timestamps do-adapter rails active_model sqlite-adapter}.each do |gem|
  gem "dm-#{gem}", :git => "https://github.com/datamapper/dm-#{gem}.git", :branch => 'release-1.2'
end

What does it do? Basically, it iterates through a bunch of strings interpolating those strings into a gem name and git repo name, and for each one, specifies branch “release 1.2” and gets the gem for that. Whoever maintains those repos has certainly been consistent!

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