Calculation in Concern module using Rails
Since you're mixing methods into ActiveRecord::Base, you might as well extend it proper:

class ActiveRecord::Base
  def self.paged(...)
    ...
  end
end


No concerns necessary; you're just extending a class. Put it into a file in config/initializers and it'll load when the app spins up.

About what you're mixing in:


default_per_page doesn't make sense as a mixin. It's not the responsibility of every ActiveRecord-derived class to have a shortcut for fetching a global config value. The methods you add to a class via mixins should make as much sense, as if you'd added them directly to just that class.
I'd place shortcuts like that in e.g. a separate helper module, and call it there.
Use Rails.configuration instead of &lt;YourAppName&gt;.config if you're aiming for reusable code.
Consider using hashes for method parameters. It makes calling your methods simpler and more descriptive.
Be consistent with your parentheses. You declaration of normalize_per_page omits the parentheses, but your declaration of normalize_page includes them. Personally, I always use parens in method declarations.
Do not access params in a model. Keep that in the controller, and only the controller; it's its job. This also means keeping the normalize_page  method (which I'd call constrain_page_number, because it doesn't normalize a page, but constrains a number) in the controller. You can simply add it to ApplicationController to have available:

# in app/application_controller.rb

protected

def constrain_page_number(number, record_count, per_page = nil)
  per_page ||= PagniationHelpers.default_per_page # our helper module
  [1, number, (record_count / per_page).floor + 1].sort[1]
end


However, in my view, your controller should send a redirect if the page number in out of bounds - don't just rewrite it behind the scenes. So you might want something like this instead:

def valid_page_number?(number, record_count, per_page = nil)
  per_page ||= PagniationHelpers.default_per_page # our helper module
  max = (record_count / per_page).floor + 1
  (1..max).cover?(number)
end

Now, something that'd might make more sense to include in every model would be a max_page method. It'd allow you to more easily check the page number in the controller.
Of course, such a method would (unlike the above) have to rely on just the default scope when counting the number of records. Still, it might be useful.


In all I end with something like this (plus the controller method above):

module PagniationHelpers
  module_function

  def default_per_page
    Rails.config.custom.default_per_page_pagination
  end
end

class ActiveRecord::Base
  class &lt;&lt; self
    def paged(page, per_page = nil)
      per_page ||= PagniationHelpers.default_per_page
      self.paginate(page: page, per_page: per_page)
    end
  end
end


Buuuut of course, it looks like you're using the will_paginate gem. In that case, forget the code above. Instead, do this in an initializer file somewhere:

WillPaginate.per_page = 10 # or whatever default you want


and in ApplicationController do:

def max_page_number(record_count, per_page = nil)
  per_page ||= WillPaginate.per_page
  (record_count / per_page).floor + 1
end

def valid_page_number?(number, record_count, per_page = nil)
  max = max_page_number(record_count, per_page)
  (1..max).cover?(number)
end

def constrain_page_number(number, record_count, per_page = nil)
  max = max_page_number(record_count, per_page)
  [1, number, max].sort[1]
end


And in your controller actions you can do something like

def index
  page = params[:page].try(:to_i) || 1
  unless valid_page_number?(page, Post.count)
    redirect_to posts_path(page: constrain_page_number(page, Post.count))
  end
  @posts = Post.paginate(:page =&gt; page)
  #...
end