Just a refactoring of code from: http://www.justinweiss.com/blog/2014/02/17/search-and-filter-rails-models-without-bloating-your-controller/

filterable.rb

module Filterable
  extend ActiveSupport::Concern

  module ClassMethods
    def filter(filtering_params)
      filtering_params.reduce(self) do |relation, (scope_name, value)|
        relation.public_send(scope_name, value) if value.present?
      end
    end
  end
end

and from a comment on the original:

This will return nil if one or more of the parameters are passed with a blank value, which means you won't be able to chain any methods after .filter in the controller, such as .page(params[:page]) if you were using a pagination gem like Kaminari, for example. The filter method should always be returning a chainable relation, in this case via .all if none of the filters are set.

Here's what it should look like:

filtering_params.reduce(self) do |relation, (scope_name, value)|
  value.present? ? relation.public_send(scope_name, value) : relation.all
end