Add specs for standalone pagination for serializers

app/serializers/pipeline_serializer.rb

@@ -12,10 +12,11 @@ class PipelineSerializer < BaseSerializer
   end
 
   def represent(resource, opts = {})
-    if paginated?
-      raise InvalidResourceError unless resource.respond_to?(:page)
-
+    if resource.is_a?(ActiveRecord::Relation)
       resource = resource.includes(project: :namespace)
+    end
+
+    if paginated?
       super(@paginator.paginate(resource), opts)
     else
       super(resource, opts)

lib/gitlab/serializer/pagination.rb

 module Gitlab
   module Serializer
     class Pagination
+      class InvalidResourceError < StandardError; end
       include ::API::Helpers::Pagination
 
       def initialize(request, response)
@@ -8,6 +9,14 @@ module Gitlab
         @response = response
       end
 
+      def paginate(resource)
+        if resource.respond_to?(:page)
+          super(resource)
+        else
+          raise InvalidResourceError
+        end
+      end
+
       private
 
       # Methods needed by `API::Helpers::Pagination`

spec/lib/gitlab/serializer/pagination_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Serializer::Pagination do
+  let(:request) { spy('request') }
+  let(:response) { spy('response') }
+
+  before do
+    allow(request)
+      .to receive(:query_parameters)
+      .and_return(params)
+  end
+
+  let(:pagination) { described_class.new(request, response) }
+
+  describe '#paginate' do
+    subject { pagination.paginate(resource) }
+
+    let(:resource) { User.all }
+    let(:params) { { page: 1, per_page: 2 } }
+
+    context 'when a multiple resources are present in relation' do
+      before { create_list(:user, 3) }
+
+      it 'correctly paginates the resource' do
+        expect(subject.count).to be 2
+      end
+
+      it 'appends relevant headers' do
+        expect(response).to receive(:[]=).with('X-Total', '3')
+        expect(response).to receive(:[]=).with('X-Total-Pages', '2')
+        expect(response).to receive(:[]=).with('X-Per-Page', '2')
+
+        subject
+      end
+    end
+
+    context 'when an invalid resource is about to be paginated' do
+      let(:resource) { create(:user) }
+
+      it 'raises error' do
+        expect { subject }.to raise_error(
+          described_class::InvalidResourceError)
+      end
+    end
+  end
+end

spec/serializers/pipeline_serializer_spec.rb

@@ -58,8 +58,8 @@ describe PipelineSerializer do
           let(:pagination) { { page: 1, per_page: 1 } }
 
           it 'raises error' do
-            expect { subject }
-              .to raise_error(PipelineSerializer::InvalidResourceError)
+            expect { subject }.to raise_error(
+              Gitlab::Serializer::Pagination::InvalidResourceError)
           end
         end
       end