pagination: fix pagination module

Previous implementation was not really compatible with active record
models. It fixes it with scope instead of methods.

app/models/paginated.rb

 module Paginated
   DEFAULT_COUNT_BY_PAGE = 10
-  # can be overwritten
-  def count_by_page
-    DEFAULT_COUNT_BY_PAGE
-  end
 
-  def paginate(params)
-    page = (params.fetch :page, 0).to_i
-    start = page * count_by_page
-    offset(start).limit(count_by_page)
+  def self.included(includer)
+    includer.scope :paginate, -> (params) {
+      count_by_page = includer.respond_to?(:count_by_page) ? includer.count_by_page : DEFAULT_COUNT_BY_PAGE
+      page = (params.fetch :page, 0).to_i
+      start = page * count_by_page
+      offset(start).limit(count_by_page)
+    }
   end
 end

test/models/paginated_test.rb

 require "test_helper"
 require_relative "../../app/models/paginated"
 
+# fake model, fake behavior, etc.
 class FakePaginated
-  include Paginated
   DATA = ("a".."z").to_a
 
   # fake inspection
-  attr_reader :data
+  def self.data
+    @@data
+  end
 
   # fake a model
-  def initialize
-    @data = DATA
+  def self.scope(scope_name, callback)
+    define_singleton_method scope_name, callback
   end
 
   # fake a model
-  def offset(start)
-    @data = @data[start..]
+  def self.reset!
+    @@data = DATA
+  end
+
+  # fake a model
+  def self.offset(start)
+    @@data = @@data[start..]
     self
   end
 
   # fake a model
-  def limit(count)
-    @data = @data[...count]
+  def self.limit(count)
+    @@data = @@data[...count]
     self
   end
 
-  def count_by_page
+  def self.count_by_page
     3
   end
+
+  include Paginated
 end
 
 class PaginatedTest < ActiveSupport::TestCase
   end
 
   test "paginated with placeholder" do
-    test_empty_params = FakePaginated.new.paginate({})
-    assert_valid_pagination test_empty_params, count: FakePaginated.new.count_by_page
+    FakePaginated.reset!
+    test_empty_params = FakePaginated.paginate({})
+    assert_valid_pagination test_empty_params, count: FakePaginated.count_by_page
     assert_equal %w[a b c], test_empty_params.data
 
-    test_valid_params = FakePaginated.new.paginate({ page: 1 })
-    assert_valid_pagination test_valid_params, count: FakePaginated.new.count_by_page
+    FakePaginated.reset!
+    test_valid_params = FakePaginated.paginate({ page: 1 })
+    assert_valid_pagination test_valid_params, count: FakePaginated.count_by_page
     assert_equal %w[d e f], test_valid_params.data
-    test_valid_params = FakePaginated.new.paginate({ page: 4 })
-    assert_valid_pagination test_valid_params, count: FakePaginated.new.count_by_page
+    FakePaginated.reset!
+    test_valid_params = FakePaginated.paginate({ page: 4 })
+    assert_valid_pagination test_valid_params, count: FakePaginated.count_by_page
     assert_equal %w[m n o], test_valid_params.data
 
-    test_invalid_params = FakePaginated.new.paginate({ page: "what" })
-    assert_valid_pagination test_invalid_params, count: FakePaginated.new.count_by_page
+    FakePaginated.reset!
+    test_invalid_params = FakePaginated.paginate({ page: "what" })
+    assert_valid_pagination test_invalid_params, count: FakePaginated.count_by_page
     assert_equal %w[a b c], test_invalid_params.data
   end
 end