Abstract over nils_last ordering

This adds a NilsLast abstraction that makes generating correct
sort-orders in Ruby simpler.

spec/graphql/resolvers/base_resolver_spec.rb

@@ -277,7 +277,7 @@ RSpec.describe Resolvers::BaseResolver do
   describe '#offset_pagination' do
     let(:instance) { resolver_instance(resolver) }
 
-    it 'is sugar for OffsetActiveRecordRelationConnection.new' do
+    it 'is sugar for OffsetPaginatedRelation.new' do
       expect(instance.offset_pagination(User.none)).to be_a(::Gitlab::Graphql::Pagination::OffsetPaginatedRelation)
     end
   end

spec/graphql/resolvers/merge_requests_resolver_spec.rb

@@ -4,6 +4,7 @@ require 'spec_helper'
 
 RSpec.describe Resolvers::MergeRequestsResolver do
   include GraphqlHelpers
+  include SortingHelper
 
   let_it_be(:project) { create(:project, :repository) }
   let_it_be(:milestone) { create(:milestone, project: project) }
@@ -40,7 +41,7 @@ RSpec.describe Resolvers::MergeRequestsResolver do
     #   AND "merge_requests"."iid" = 1 ORDER BY "merge_requests"."id" DESC
     # SELECT "projects".* FROM "projects" WHERE "projects"."id" = 2
     # SELECT "project_features".* FROM "project_features" WHERE "project_features"."project_id" = 2
-    let(:queries_per_project) { 4 }
+    let(:queries_per_project) { 3 }
 
     context 'no arguments' do
       it 'returns all merge requests' do
@@ -250,17 +251,17 @@ RSpec.describe Resolvers::MergeRequestsResolver do
         it 'sorts merge requests ascending' do
           expect(resolve_mr(project, sort: :merged_at_asc))
             .to match_array(mrs)
-            .and be_sorted(->(mr) { [merged_at(mr, 1.year.from_now).to_i, -mr.id] })
+            .and be_sorted(->(mr) { [merged_at(mr), -mr.id] })
         end
 
         it 'sorts merge requests descending' do
           expect(resolve_mr(project, sort: :merged_at_desc))
             .to match_array(mrs)
-            .and be_sorted(->(mr) { [-merged_at(mr).to_i, -mr.id] })
+            .and be_sorted(->(mr) { [-merged_at(mr), -mr.id] })
         end
 
-        def merged_at(mr, when_nil = nil)
-          mr.metrics.merged_at || when_nil
+        def merged_at(mr)
+          nils_last(mr.metrics.merged_at)
         end
 
         context 'when label filter is given and the optimized_issuable_label_filter feature flag is off' do

spec/support/helpers/sorting_helper.rb

@@ -17,4 +17,35 @@ module SortingHelper
       click_link value
     end
   end
+
+  def nils_last(value)
+    NilsLast.new(value)
+  end
+
+  class NilsLast
+    include Comparable
+
+    attr_reader :value
+    delegate :==, :eql?, :hash, to: :value
+
+    def initialize(value)
+      @value = value
+      @reverse = false
+    end
+
+    def <=>(other)
+      return unless other.is_a?(self.class)
+      return 0 if value.nil? && other.value.nil?
+      return 1 if value.nil?
+      return -1 if other.value.nil?
+
+      int = value <=> other.value
+      @reverse ? -int : int
+    end
+
+    def -@
+      @reverse = true
+      self
+    end
+  end
 end