Allow to order epics by start or end date

This implements sorting used on epics roadmap page as described
here:
https://gitlab.com/gitlab-org/gitlab-ee/issues/3559#sorting-logic-of-epics

It sorts epics by start_date, if an epic does not have start_date set,
it uses end_date in sorting.

Related #3559

ee/app/models/ee/epic.rb

@@ -14,6 +14,13 @@ module EE
       has_many :epic_issues
 
       validates :group, presence: true
+
+      scope :order_start_or_end_date_asc, -> do
+        # mysql returns null values first in opposite to postgres which
+        # returns them last by default
+        nulls_first = ::Gitlab::Database.postgresql? ? 'NULLS FIRST' : ''
+        reorder("COALESCE(start_date, end_date) ASC #{nulls_first}")
+      end
     end
 
     module ClassMethods
@@ -54,6 +61,14 @@ module EE
           )
         }x
       end
+
+      def order_by(method)
+        if method.to_s == 'start_or_end_date'
+          order_start_or_end_date_asc
+        else
+          super
+        end
+      end
     end
 
     def assignees

spec/ee/spec/models/epic_spec.rb

@@ -23,6 +23,19 @@ describe Epic do
     it { is_expected.to include_module(InternalId) }
   end
 
+  describe '.order_start_or_end_date_asc' do
+    let(:group) { create(:group) }
+
+    it 'returns epics sorted by start or end date' do
+      epic1 = create(:epic, group: group, start_date: 7.days.ago, end_date: 3.days.ago)
+      epic2 = create(:epic, group: group, start_date: 3.days.ago)
+      epic3 = create(:epic, group: group, end_date: 5.days.ago)
+      epic4 = create(:epic, group: group)
+
+      expect(described_class.order_start_or_end_date_asc).to eq([epic4, epic1, epic3, epic2])
+    end
+  end
+
   describe '#issues' do
     let(:user) { create(:user) }
     let(:group) { create(:group, :private) }