[MODEL] Added the offset method to the "will_paginate" integration

Closes elastic#135

Related: elastic#131, elastic#135, elastic#175

Author: balexand

elasticsearch-model/lib/elasticsearch/model/response/pagination.rb

@@ -102,11 +102,15 @@ def self.included(base)
 
             # Include the paging methods in results and records
             #
-            methods = [:current_page, :per_page, :total_entries, :total_pages, :previous_page, :next_page, :out_of_bounds?]
+            methods = [:current_page, :offset, :per_page, :total_entries, :total_pages, :previous_page, :next_page, :out_of_bounds?]
             Elasticsearch::Model::Response::Results.__send__ :delegate, *methods, to: :response
             Elasticsearch::Model::Response::Records.__send__ :delegate, *methods, to: :response
           end
 
+          def offset
+            (current_page - 1) * per_page
+          end
+
           # Main pagination method
           #
           # @example

elasticsearch-model/test/unit/response_pagination_will_paginate_test.rb

@@ -30,6 +30,7 @@ class WillPaginateResponse < Elasticsearch::Model::Response::Response
       @expected_methods = [
         # methods needed by WillPaginate::CollectionMethods
         :current_page,
+        :offset,
         :per_page,
         :total_entries,
 
@@ -66,6 +67,13 @@ class WillPaginateResponse < Elasticsearch::Model::Response::Response
       end
     end
 
+    context "#offset method" do
+      should "calculate offset using current_page and per_page" do
+        @response.per_page(3).page(3)
+        assert_equal 6, @response.offset
+      end
+    end
+
     context "#paginate method" do
       should "set from/size using defaults" do
         @response.klass.client