Undefined Method `per' for #<ActiveRecord::Relation:0x.....>
This is because Active Admin uses kaminari for pagination. To fix this error make sure will_paginate and kaminari don't collide, as per : using active admin with will_paginate by adding some config to kaminari:
# create config/initializers/kaminari.rb Kaminari.configure do |config| config.page_method_name = :per_page_kaminari end
Google is likely to lead you to a github discussion that tells you to use a very specific version of will_paginate, but obviously the solution above is better.