Improve error message and minor clean up

Improve error message to display the type of relation the request has
failed for, to be clear to users that limitations are specific to the
type of results requested

lib/api/helpers/pagination_strategies.rb

@@ -11,7 +11,7 @@ module API
         end
       end
 
-      def paginator(relation, request_scope)
+      def paginator(relation, request_scope = nil)
         return keyset_paginator(relation) if keyset_pagination_enabled?
 
         offset_paginator(relation, request_scope)
@@ -31,8 +31,9 @@ module API
       def offset_paginator(relation, request_scope)
         offset_limit = limit_for_scope(request_scope)
         if Gitlab::Pagination::Keyset.available_for_type?(relation) && offset_limit_exceeded?(offset_limit)
-          return error!("Offset pagination has a maximum allowed offset of #{offset_limit}, " \
-            "remaining records can be retrieved using keyset pagination.", 405)
+          return error!("Offset pagination has a maximum allowed offset of #{offset_limit} " \
+            "for requests that return objects of type #{relation.klass}. " \
+            "Remaining records can be retrieved using keyset pagination.", 405)
         end
 
         Gitlab::Pagination::OffsetPagination.new(self)

lib/gitlab/pagination/keyset.rb

@@ -3,8 +3,12 @@
 module Gitlab
   module Pagination
     module Keyset
+      SUPPORTED_TYPES = [
+        Project
+      ].freeze
+
       def self.available_for_type?(relation)
-        relation.klass == Project
+        SUPPORTED_TYPES.include?(relation.klass)
       end
 
       def self.available?(request_context, relation)

spec/lib/api/helpers/pagination_strategies_spec.rb

@@ -6,7 +6,7 @@ describe API::Helpers::PaginationStrategies do
   subject { Class.new.include(described_class).new }
 
   let(:expected_result) { double("result") }
-  let(:relation) { double("relation") }
+  let(:relation) { double("relation", klass: "SomeClass") }
   let(:params) { {} }
 
   before do
@@ -90,7 +90,7 @@ describe API::Helpers::PaginationStrategies do
             it 'renders a 405 error' do
               expect(subject).to receive(:error!).with(/maximum allowed offset/, 405)
 
-              subject.paginator(relation, nil)
+              subject.paginator(relation)
             end
           end
 
@@ -100,7 +100,7 @@ describe API::Helpers::PaginationStrategies do
             it 'delegates to OffsetPagination' do
               expect(Gitlab::Pagination::OffsetPagination).to receive(:new).with(subject).and_return(paginator)
 
-              expect(subject.paginator(relation, nil)).to eq(paginator)
+              expect(subject.paginator(relation)).to eq(paginator)
             end
           end
         end
@@ -116,7 +116,7 @@ describe API::Helpers::PaginationStrategies do
         it 'delegates to OffsetPagination' do
           expect(Gitlab::Pagination::OffsetPagination).to receive(:new).with(subject).and_return(paginator)
 
-          expect(subject.paginator(relation, nil)).to eq(paginator)
+          expect(subject.paginator(relation)).to eq(paginator)
         end
       end
     end
@@ -138,7 +138,7 @@ describe API::Helpers::PaginationStrategies do
         end
 
         it 'delegates to Pager' do
-          expect(subject.paginator(relation, nil)).to eq(pager)
+          expect(subject.paginator(relation)).to eq(pager)
         end
       end
 
@@ -150,7 +150,7 @@ describe API::Helpers::PaginationStrategies do
         it 'renders a 501 error' do
           expect(subject).to receive(:error!).with(/not yet available/, 405)
 
-          subject.paginator(relation, nil)
+          subject.paginator(relation)
         end
       end
     end