exchange pagination class to inside json_api scope

lib/active_model/serializer/adapter/json_api.rb

@@ -1,4 +1,5 @@
 require 'active_model/serializer/adapter/json_api/fragment_cache'
+require 'active_model/serializer/adapter/json_api/pagination_links'
 
 module ActiveModel
   class Serializer
@@ -27,6 +28,8 @@ module ActiveModel
                 @hash[:included] |= result[:included]
               end
             end
+
+            include_pagination_links if serializer.pagination
           else
             @hash[:data] = attributes_for_serializer(serializer, options)
             add_resource_relationships(@hash[:data], serializer)
@@ -157,6 +160,20 @@ module ActiveModel
             end
           end
         end
+
+        def include_pagination_links
+          return if page_links.empty?
+
+          links? ? @hash[:links].merge!(page_links) : @hash[:links] = page_links
+        end
+
+        def page_links
+          @links ||= JsonApi::PaginationLinks.new(serializer.resource).page_links
+        end
+
+        def links?
+          !@hash[:links].nil?
+        end
       end
     end
   end

lib/active_model/serializer/adapter/json_api/pagination_links.rb

@@ -0,0 +1,52 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi < Adapter
+        class PaginationLinks
+          FIRST_PAGE = 1
+
+          attr_reader :collection
+
+          def initialize(collection)
+            raise_unless_any_gem_installed
+            @collection = collection
+          end
+
+          def page_links
+            build_links
+          end
+
+          private
+
+          def build_links
+            pages_from.each_with_object({}) do |(key, value), hash|
+              hash[key] = "?page=#{value}&per_page=#{collection.size}"
+            end
+          end
+
+          def pages_from
+            return {} if collection.total_pages == FIRST_PAGE
+
+            {}.tap do |pages|
+              unless collection.current_page == FIRST_PAGE
+                pages[:first] = FIRST_PAGE
+                pages[:prev]  = collection.current_page - FIRST_PAGE
+              end
+
+              unless collection.current_page == collection.total_pages
+                pages[:next] = collection.current_page + FIRST_PAGE
+                pages[:last] = collection.total_pages
+              end
+            end
+          end
+
+          def raise_unless_any_gem_installed
+            return if defined?(WillPaginate) || defined?(Kaminari)
+            raise "AMS relies on either Kaminari or WillPaginate." +
+              "Please install either dependency by adding one of those to your Gemfile"
+          end
+        end
+      end
+    end
+  end
+end