Still using will_paginate is also okay.
This thread has same issue: undefined method `paginate' for Array on Rails 3 with mongoid
The main point to fix the error is to add this line before the controller call paginate library:
require 'will_paginate/array'
It should be added to default config file if you use mongoid for the whole project.
Hope the explanation helpful.
Reference from origin gem source: https://github.com/mislav/will_paginate/wiki/Backwards-incompatibility at "WillPaginate::Collection" section.
P/S: this is just a work around if your query is not very large. If you want better performance, let's try mongoid-pagination gem, custom will_pagination gem or another pagination gem which supported Mongoid like kaminari.