Currently loading rails_admin with latest mongoid will throw an error:
"Need to map field Moped::BSON::ObjectId for field name _id ..."
A dirty fix that only adds the field mapping is available in my fork: http://github.com/lieldulev/rails_admin/commit/bc9790beeaeee30c31d31cf1e495a723d2ec1713
Keep up with the recent changes of Mongoid master. Fixes #1136
I've just finished my ongoing work to support the latest Mongoid and pushed to master!